BTRFS(5) BTRFS BTRFS(5)

NAME
btrfs - topics about the BTRFS filesystem (mount options, supported
file attributes and other)

DESCRIPTION
This document describes topics related to BTRFS that are not specific
to the tools. Currently covers:

1. mount options

2. filesystem features

3. checksum algorithms

4. compression

5. sysfs interface

6. filesystem exclusive operations

7. filesystem limits

8. bootloader support

9. file attributes

10. zoned mode

11. control device

12. filesystems with multiple block group profiles

13. seeding device

14. RAID56 status and recommended practices

15. glossary

16. storage model, hardware considerations

MOUNT OPTIONS
BTRFS SPECIFIC MOUNT OPTIONS
This section describes mount options specific to BTRFS. For the
generic mount options please refer to mount(8) manual page and also see
the section with BTRFS specifics below. The options are sorted
alphabetically (discarding the no prefix).

NOTE:
Most mount options apply to the whole filesystem and only options in
the first mounted subvolume will take effect. This is due to lack of
implementation and may change in the future. This means that (for
example) you can't set per-subvolume nodatacow, nodatasum, or
compress using mount options. This should eventually be fixed, but
it has proved to be difficult to implement correctly within the
Linux VFS framework.

Mount options are processed in order, only the last occurrence of an
option takes effect and may disable other options due to constraints
(see e.g. nodatacow and compress). The output of mount command shows
which options have been applied.

acl, noacl
(default: on)

Enable/disable support for POSIX Access Control Lists (ACLs).
See the acl(5) manual page for more information about ACLs.

The support for ACL is build-time configurable
(BTRFS_FS_POSIX_ACL) and mount fails if acl is requested but the
feature is not compiled in.

autodefrag, noautodefrag
(since: 3.0, default: off)

Enable automatic file defragmentation. When enabled, small
random writes into files (in a range of tens of kilobytes,
currently it's 64KiB) are detected and queued up for the
defragmentation process. May not be well suited for large
database workloads.

The read latency may increase due to reading the adjacent blocks
that make up the range for defragmentation, successive write
will merge the blocks in the new location.

WARNING:
Defragmenting with Linux kernel versions < 3.9 or >= 3.14-rc2
as well as with Linux stable kernel versions >= 3.10.31, >=
3.12.12 or >= 3.13.4 will break up the reflinks of COW data
(for example files copied with cp --reflink, snapshots or
de-duplicated data). This may cause considerable increase of
space usage depending on the broken up reflinks.

barrier, nobarrier
(default: on)

Ensure that all IO write operations make it through the device
cache and are stored permanently when the filesystem is at its
consistency checkpoint. This typically means that a flush
command is sent to the device that will synchronize all pending
data and ordinary metadata blocks, then writes the superblock
and issues another flush.

The write flushes incur a slight hit and also prevent the IO
block scheduler to reorder requests in a more effective way.
Disabling barriers gets rid of that penalty but will most
certainly lead to a corrupted filesystem in case of a crash or
power loss. The ordinary metadata blocks could be yet unwritten
at the time the new superblock is stored permanently, expecting
that the block pointers to metadata were stored permanently
before.

On a device with a volatile battery-backed write-back cache, the
nobarrier option will not lead to filesystem corruption as the
pending blocks are supposed to make it to the permanent storage.

clear_cache
Force clearing and rebuilding of the free space cache if
something has gone wrong.

For free space cache v1, this only clears (and, unless
nospace_cache is used, rebuilds) the free space cache for block
groups that are modified while the filesystem is mounted with
that option. To actually clear an entire free space cache v1,
see btrfs check --clear-space-cache v1.

For free space cache v2, this clears the entire free space
cache. To do so without requiring to mounting the filesystem,
see btrfs check --clear-space-cache v2.

See also: space_cache.

commit=<seconds>
(since: 3.12, default: 30)

Set the interval of periodic transaction commit when data are
synchronized to permanent storage. Higher interval values lead
to larger amount of unwritten data to accumulate in memory,
which has obvious consequences when the system crashes. The
upper bound is not forced, but a warning is printed if it's more
than 300 seconds (5 minutes). Use with care.

The periodic commit is not the only mechanism to do the
transaction commit, this can also happen by explicit sync or
indirectly by other commands that affect the global filesystem
state or internal kernel mechanisms that flush based on various
thresholds or policies (e.g. cgroups).

compress, compress=<type[:level]>, compress-force,
compress-force=<type[:level]>
(default: off, level support since: 5.1)

Control BTRFS file data compression. Type may be specified as
zlib, lzo, zstd or no (for no compression, used for remounting).
If no type is specified, zlib is used. If compress-force is
specified, then compression will always be attempted, but the
data may end up uncompressed if the compression would make them
larger.

Both zlib and zstd (since version 5.1) expose the compression
level as a tunable knob with higher levels trading speed and
memory (zstd) for higher compression ratios. This can be set by
appending a colon and the desired level. ZLIB accepts the range
[1, 9] and ZSTD accepts [1, 15]. If no level is set, both
currently use a default level of 3. The value 0 is an alias for
the default level.

Otherwise some simple heuristics are applied to detect an
incompressible file. If the first blocks written to a file are
not compressible, the whole file is permanently marked to skip
compression. As this is too simple, the compress-force is a
workaround that will compress most of the files at the cost of
some wasted CPU cycles on failed attempts. Since kernel 4.15, a
set of heuristic algorithms have been improved by using
frequency sampling, repeated pattern detection and Shannon
entropy calculation to avoid that.

NOTE:
If compression is enabled, nodatacow and nodatasum are
disabled.

datacow, nodatacow
(default: on)

Enable data copy-on-write for newly created files. Nodatacow
implies nodatasum, and disables compression. All files created
under nodatacow are also set the NOCOW file attribute (see
chattr(1)).

NOTE:
If nodatacow or nodatasum are enabled, compression is
disabled.

Updates in-place improve performance for workloads that do
frequent overwrites, at the cost of potential partial writes, in
case the write is interrupted (system crash, device failure).

datasum, nodatasum
(default: on)

Enable data checksumming for newly created files. Datasum
implies datacow, i.e. the normal mode of operation. All files
created under nodatasum inherit the "no checksums" property,
however there's no corresponding file attribute (see chattr(1)).

NOTE:
If nodatacow or nodatasum are enabled, compression is
disabled.

There is a slight performance gain when checksums are turned
off, the corresponding metadata blocks holding the checksums do
not need to updated. The cost of checksumming of the blocks in
memory is much lower than the IO, modern CPUs feature hardware
support of the checksumming algorithm.

degraded
(default: off)

Allow mounts with fewer devices than the RAID profile
constraints require. A read-write mount (or remount) may fail
when there are too many devices missing, for example if a stripe
member is completely missing from RAID0.

Since 4.14, the constraint checks have been improved and are
verified on the chunk level, not at the device level. This
allows degraded mounts of filesystems with mixed RAID profiles
for data and metadata, even if the device number constraints
would not be satisfied for some of the profiles.

Example: metadata -- raid1, data -- single, devices -- /dev/sda,
/dev/sdb

Suppose the data are completely stored on sda, then missing sdb
will not prevent the mount, even if 1 missing device would
normally prevent (any) single profile to mount. In case some of
the data chunks are stored on sdb, then the constraint of
single/data is not satisfied and the filesystem cannot be
mounted.

device=<devicepath>
Specify a path to a device that will be scanned for BTRFS
filesystem during mount. This is usually done automatically by a
device manager (like udev) or using the btrfs device scan
command (e.g. run from the initial ramdisk). In cases where this
is not possible the device mount option can help.

NOTE:
Booting e.g. a RAID1 system may fail even if all filesystem's
device paths are provided as the actual device nodes may not
be discovered by the system at that point.

discard, discard=sync, discard=async, nodiscard
(default: async when devices support it since 6.2, async support
since: 5.6)

Enable discarding of freed file blocks. This is useful for
SSD/NVMe devices, thinly provisioned LUNs, or virtual machine
images; however, every storage layer must support discard for it
to work.

In the synchronous mode (sync or without option value), lack of
asynchronous queued TRIM on the backing device TRIM can severely
degrade performance, because a synchronous TRIM operation will
be attempted instead. Queued TRIM requires SATA devices with
chipsets revision newer than 3.1 and devices.

The asynchronous mode (async) gathers extents in larger chunks
before sending them to the devices for TRIM. The overhead and
performance impact should be negligible compared to the previous
mode and it's supposed to be the preferred mode if needed.

If it is not necessary to immediately discard freed blocks, then
the fstrim tool can be used to discard all free blocks in a
batch. Scheduling a TRIM during a period of low system activity
will prevent latent interference with the performance of other
operations. Also, a device may ignore the TRIM command if the
range is too small, so running a batch discard has a greater
probability of actually discarding the blocks.

enospc_debug, noenospc_debug
(default: off)

Enable verbose output for some ENOSPC conditions. It's safe to
use but can be noisy if the system reaches near-full state.

fatal_errors=<action>
(since: 3.4, default: bug)

Action to take when encountering a fatal error.

bug BUG() on a fatal error, the system will stay in the
crashed state and may be still partially usable, but
reboot is required for full operation

panic panic() on a fatal error, depending on other system
configuration, this may be followed by a reboot. Please
refer to the documentation of kernel boot parameters,
e.g. panic, oops or crashkernel.

flushoncommit, noflushoncommit
(default: off)

This option forces any data dirtied by a write in a prior
transaction to commit as part of the current commit, effectively
a full filesystem sync.

This makes the committed state a fully consistent view of the
file system from the application's perspective (i.e. it includes
all completed file system operations). This was previously the
behavior only when a snapshot was created.

When off, the filesystem is consistent but buffered writes may
last more than one transaction commit.

fragment=<type>
(depends on compile-time option CONFIG_BTRFS_DEBUG, since: 4.4,
default: off)

A debugging helper to intentionally fragment given type of block
groups. The type can be data, metadata or all. This mount option
should not be used outside of debugging environments and is not
recognized if the kernel config option CONFIG_BTRFS_DEBUG is not
enabled.

nologreplay
(default: off, even read-only)

The tree-log contains pending updates to the filesystem until
the full commit. The log is replayed on next mount, this can be
disabled by this option. See also treelog. Note that
nologreplay is the same as norecovery.

WARNING:
Currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, mount also with
nologreplay.

max_inline=<bytes>
(default: min(2048, page size) )

Specify the maximum amount of space, that can be inlined in a
metadata b-tree leaf. The value is specified in bytes,
optionally with a K suffix (case insensitive). In practice,
this value is limited by the filesystem block size (named
sectorsize at mkfs time), and memory page size of the system. In
case of sectorsize limit, there's some space unavailable due to
b-tree leaf headers. For example, a 4KiB sectorsize, maximum
size of inline data is about 3900 bytes.

Inlining can be completely turned off by specifying 0. This will
increase data block slack if file sizes are much smaller than
block size but will reduce metadata consumption in return.

NOTE:
The default value has changed to 2048 in kernel 4.6.

metadata_ratio=<value>
(default: 0, internal logic)

Specifies that 1 metadata chunk should be allocated after every
value data chunks. Default behaviour depends on internal logic,
some percent of unused metadata space is attempted to be
maintained but is not always possible if there's not enough
space left for chunk allocation. The option could be useful to
override the internal logic in favor of the metadata allocation
if the expected workload is supposed to be metadata intense
(snapshots, reflinks, xattrs, inlined files).

norecovery
(since: 4.5, default: off)

Do not attempt any data recovery at mount time. This will
disable logreplay and avoids other write operations. Note that
this option is the same as nologreplay.

NOTE:
The opposite option recovery used to have different meaning
but was changed for consistency with other filesystems, where
norecovery is used for skipping log replay. BTRFS does the
same and in general will try to avoid any write operations.

rescan_uuid_tree
(since: 3.12, default: off)

Force check and rebuild procedure of the UUID tree. This should
not normally be needed. Alternatively the tree can be cleared
from userspace by command btrfs rescue clear-uuid-tree and then
it will be automatically rebuilt in kernel (the mount option is
not needed in that case).

rescue (since: 5.9)

Modes allowing mount with damaged filesystem structures, all
requires the filesystem to be mounted read-only and doesn't
allow remount to read-write. This is supposed to provide
unified and more fine grained tuning of errors that affect
filesystem operation.

o usebackuproot (since 5.9)

Try to use backup root slots inside super block. Replaces
standalone option usebackuproot

o nologreplay (since 5.9)

Do not replay any dirty logs. Replaces standalone option
nologreplay

o ignorebadroots, ibadroots (since: 5.11)

Ignore bad tree roots, greatly improve the chance for data
salvage.

o ignoredatacsums, idatacsums (since: 5.11)

Ignore data checksum verification.

o ignoremetacsums, imetacsums (since 6.12)

Ignore metadata checksum verification, useful for interrupted
checksum conversion.

o all (since: 5.9)

Enable all supported rescue options.

skip_balance
(since: 3.3, default: off)

Skip automatic resume of an interrupted balance operation. The
operation can later be resumed with btrfs balance resume, or the
paused state can be removed with btrfs balance cancel. The
default behaviour is to resume an interrupted balance
immediately after the filesystem is mounted.

space_cache, space_cache=<version>, nospace_cache
(nospace_cache since: 3.2, space_cache=v1 and space_cache=v2
since 4.5, default: space_cache=v2)

Options to control the free space cache. The free space cache
greatly improves performance when reading block group free space
into memory. However, managing the space cache consumes some
resources, including a small amount of disk space.

There are two implementations of the free space cache. The
original one, referred to as v1, used to be a safe default but
has been superseded by v2. The v1 space cache can be disabled
at mount time with nospace_cache without clearing.

On very large filesystems (many terabytes) and certain
workloads, the performance of the v1 space cache may degrade
drastically. The v2 implementation, which adds a new b-tree
called the free space tree, addresses this issue. Once enabled,
the v2 space cache will always be used and cannot be disabled
unless it is cleared. Use clear_cache,space_cache=v1 or
clear_cache,nospace_cache to do so. If v2 is enabled, and v1
space cache will be cleared (at the first mount) and kernels
without v2 support will only be able to mount the filesystem in
read-only mode. On an unmounted filesystem the caches (both
versions) can be cleared by "btrfs check --clear-space-cache".

The btrfs-check(8) and :doc:`mkfs.btrfs commands have full v2
free space cache support since v4.19.

If a version is not explicitly specified, the default
implementation will be chosen, which is v2.

ssd, ssd_spread, nossd, nossd_spread
(default: SSD autodetected)

Options to control SSD allocation schemes. By default, BTRFS
will enable or disable SSD optimizations depending on status of
a device with respect to rotational or non-rotational type. This
is determined by the contents of
/sys/block/DEV/queue/rotational). If it is 0, the ssd option is
turned on. The option nossd will disable the autodetection.

The optimizations make use of the absence of the seek penalty
that's inherent for the rotational devices. The blocks can be
typically written faster and are not offloaded to separate
threads.

NOTE:
Since 4.14, the block layout optimizations have been dropped.
This used to help with first generations of SSD devices.
Their FTL (flash translation layer) was not effective and the
optimization was supposed to improve the wear by better
aligning blocks. This is no longer true with modern SSD
devices and the optimization had no real benefit. Furthermore
it caused increased fragmentation. The layout tuning has been
kept intact for the option ssd_spread.

The ssd_spread mount option attempts to allocate into bigger and
aligned chunks of unused space, and may perform better on
low-end SSDs. ssd_spread implies ssd, enabling all other SSD
heuristics as well. The option nossd will disable all SSD
options while nossd_spread only disables ssd_spread.

subvol=<path>
Mount subvolume from path rather than the toplevel subvolume.
The path is always treated as relative to the toplevel
subvolume. This mount option overrides the default subvolume
set for the given filesystem.

subvolid=<subvolid>
Mount subvolume specified by a subvolid number rather than the
toplevel subvolume. You can use btrfs subvolume list of btrfs
subvolume show to see subvolume ID numbers. This mount option
overrides the default subvolume set for the given filesystem.

NOTE:
If both subvolid and subvol are specified, they must point at
the same subvolume, otherwise the mount will fail.

thread_pool=<number>
(default: min(NRCPUS + 2, 8) )

The number of worker threads to start. NRCPUS is number of
on-line CPUs detected at the time of mount. Small number leads
to less parallelism in processing data and metadata, higher
numbers could lead to a performance hit due to increased locking
contention, process scheduling, cache-line bouncing or costly
data transfers between local CPU memories.

treelog, notreelog
(default: on)

Enable the tree logging used for fsync and O_SYNC writes. The
tree log stores changes without the need of a full filesystem
sync. The log operations are flushed at sync and transaction
commit. If the system crashes between two such syncs, the
pending tree log operations are replayed during mount.

WARNING:
Currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, also mount with
nologreplay.

The tree log could contain new files/directories, these would
not exist on a mounted filesystem if the log is not replayed.

usebackuproot
(since: 4.6, default: off)

Enable autorecovery attempts if a bad tree root is found at
mount time. Currently this scans a backup list of several
previous tree roots and tries to use the first readable. This
can be used with read-only mounts as well.

NOTE:
This option has replaced recovery which has been deprecated.

user_subvol_rm_allowed
(default: off)

Allow subvolumes to be deleted by their respective owner.
Otherwise, only the root user can do that.

NOTE:
Historically, any user could create a snapshot even if he was
not owner of the source subvolume, the subvolume deletion has
been restricted for that reason. The subvolume creation has
been restricted but this mount option is still required. This
is a usability issue. Since 4.18, the rmdir(2) syscall can
delete an empty subvolume just like an ordinary directory.
Whether this is possible can be detected at runtime, see
rmdir_subvol feature in FILESYSTEM FEATURES.

DEPRECATED MOUNT OPTIONS
List of mount options that have been removed, kept for backward
compatibility.

recovery
(since: 3.2, default: off, deprecated since: 4.5)

NOTE:
This option has been replaced by usebackuproot and should not
be used but will work on 4.5+ kernels.

inode_cache, noinode_cache
(removed in: 5.11, since: 3.0, default: off)

NOTE:
The functionality has been removed in 5.11, any stale data
created by previous use of the inode_cache option can be
removed by btrfs rescue clear-ino-cache.

check_int, check_int_data, check_int_print_mask=<value>
(removed in: 6.7, since: 3.0, default: off)

These debugging options control the behavior of the integrity
checking module (the BTRFS_FS_CHECK_INTEGRITY config option
required). The main goal is to verify that all blocks from a
given transaction period are properly linked.

check_int enables the integrity checker module, which examines
all block write requests to ensure on-disk consistency, at a
large memory and CPU cost.

check_int_data includes extent data in the integrity checks, and
implies the check_int option.

check_int_print_mask takes a bit mask of BTRFSIC_PRINT_MASK_*
values as defined in fs/btrfs/check-integrity.c, to control the
integrity checker module behavior.

See comments at the top of fs/btrfs/check-integrity.c for more
information.

NOTES ON GENERIC MOUNT OPTIONS
Some of the general mount options from mount(8) that affect BTRFS and
are worth mentioning.

context
The context refers to the SELinux contexts and policy
definitions passed as mount options. This works properly since
version v6.8 (because the mount option parser of BTRFS was
ported to new API that also understood the options).

noatime
under read intensive work-loads, specifying noatime
significantly improves performance because no new access time
information needs to be written. Without this option, the
default is relatime, which only reduces the number of inode
atime updates in comparison to the traditional strictatime. The
worst case for atime updates under relatime occurs when many
files are read whose atime is older than 24 h and which are
freshly snapshotted. In that case the atime is updated and COW
happens - for each file - in bulk. See also
https://lwn.net/Articles/499293/ - Atime and btrfs: a bad
combination? (LWN, 2012-05-31).

Note that noatime may break applications that rely on atime
uptimes like the venerable Mutt (unless you use maildir
mailboxes).

FILESYSTEM FEATURES
The basic set of filesystem features gets extended over time. The
backward compatibility is maintained and the features are optional,
need to be explicitly asked for so accidental use will not create
incompatibilities.

There are several classes and the respective tools to manage the
features:

at mkfs time only
This is namely for core structures, like the b-tree nodesize or
checksum algorithm, see mkfs.btrfs(8) for more details.

after mkfs, on an unmounted filesystem
Features that may optimize internal structures or add new
structures to support new functionality, see btrfstune(8). The
command btrfs inspect-internal dump-super /dev/sdx will dump a
superblock, you can map the value of incompat_flags to the
features listed below

after mkfs, on a mounted filesystem
The features of a filesystem (with a given UUID) are listed in
/sys/fs/btrfs/UUID/features/, one file per feature. The status
is stored inside the file. The value 1 is for enabled and
active, while 0 means the feature was enabled at mount time but
turned off afterwards.

Whether a particular feature can be turned on a mounted
filesystem can be found in the directory
/sys/fs/btrfs/features/, one file per feature. The value 1 means
the feature can be enabled.

List of features (see also mkfs.btrfs(8) section FILESYSTEM FEATURES):

big_metadata
(since: 3.4)

the filesystem uses nodesize for metadata blocks, this can be
bigger than the page size

block_group_tree
(since: 6.1)

block group item representation using a dedicated b-tree, this
can greatly reduce mount time for large filesystems

compress_lzo
(since: 2.6.38)

the lzo compression has been used on the filesystem, either as a
mount option or via btrfs filesystem defrag.

compress_zstd
(since: 4.14)

the zstd compression has been used on the filesystem, either as
a mount option or via btrfs filesystem defrag.

default_subvol
(since: 2.6.34)

the default subvolume has been set on the filesystem

extended_iref
(since: 3.7)

increased hardlink limit per file in a directory to 65536, older
kernels supported a varying number of hardlinks depending on the
sum of all file name sizes that can be stored into one metadata
block

free_space_tree
(since: 4.5)

free space representation using a dedicated b-tree, successor of
v1 space cache

metadata_uuid
(since: 5.0)

the main filesystem UUID is the metadata_uuid, which stores the
new UUID only in the superblock while all metadata blocks still
have the UUID set at mkfs time, see btrfstune(8) for more

mixed_backref
(since: 2.6.31)

the last major disk format change, improved backreferences, now
default

mixed_groups
(since: 2.6.37)

mixed data and metadata block groups, i.e. the data and metadata
are not separated and occupy the same block groups, this mode is
suitable for small volumes as there are no constraints how the
remaining space should be used (compared to the split mode,
where empty metadata space cannot be used for data and vice
versa)

on the other hand, the final layout is quite unpredictable and
possibly highly fragmented, which means worse performance

no_holes
(since: 3.14)

improved representation of file extents where holes are not
explicitly stored as an extent, saves a few percent of metadata
if sparse files are used

raid1c34
(since: 5.5)

extended RAID1 mode with copies on 3 or 4 devices respectively

raid_stripe_tree
(since: 6.7)

a separate tree for tracking file extents on RAID profiles

RAID56 (since: 3.9)

the filesystem contains or contained a RAID56 profile of block
groups

rmdir_subvol
(since: 4.18)

indicate that rmdir(2) syscall can delete an empty subvolume
just like an ordinary directory. Note that this feature only
depends on the kernel version.

skinny_metadata
(since: 3.10)

reduced-size metadata for extent references, saves a few percent
of metadata

send_stream_version
(since: 5.10)

number of the highest supported send stream version

simple_quota
(since: 6.7)

simplified quota accounting

supported_checksums
(since: 5.5)

list of checksum algorithms supported by the kernel module, the
respective modules or built-in implementing the algorithms need
to be present to mount the filesystem, see section CHECKSUM
ALGORITHMS.

supported_sectorsizes
(since: 5.13)

list of values that are accepted as sector sizes (mkfs.btrfs
--sectorsize) by the running kernel

supported_rescue_options
(since: 5.11)

list of values for the mount option rescue that are supported by
the running kernel, see btrfs(5)

zoned (since: 5.12)

zoned mode is allocation/write friendly to host-managed zoned
devices, allocation space is partitioned into fixed-size zones
that must be updated sequentially, see section ZONED MODE

SWAPFILE SUPPORT
A swapfile, when active, is a file-backed swap area. It is supported
since kernel 5.0. Use swapon(8) to activate it, until then
(respectively again after deactivating it with swapoff(8)) it's just a
normal file (with NODATACOW set), for which the special restrictions
for active swapfiles don't apply.

There are some limitations of the implementation in BTRFS and Linux
swap subsystem:

o filesystem - must be only single device

o filesystem - must have only single data profile

o subvolume - cannot be snapshotted if it contains any active swapfiles

o swapfile - must be preallocated (i.e. no holes)

o swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)

The limitations come namely from the COW-based design and mapping layer
of blocks that allows the advanced features like relocation and
multi-device filesystems. However, the swap subsystem expects simpler
mapping and no background changes of the file block location once
they've been assigned to swap. The constraints mentioned above (single
device and single profile) are related to the swapfile itself, i.e. the
extents and their placement. It is possible to create swapfile on
multi-device filesystem as long as the extents are on one device but
this cannot be affected by user and depends on free space fragmentation
and available unused space for new chunks.

With active swapfiles, the following whole-filesystem operations will
skip swapfile extents or may fail:

o balance - block groups with extents of any active swapfiles are
skipped and reported, the rest will be processed normally

o resize grow - unaffected

o resize shrink - works as long as the extents of any active swapfiles
are outside of the shrunk range

o device add - if the new devices do not interfere with any already
active swapfiles this operation will work, though no new swapfile can
be activated afterwards

o device delete - if the device has been added as above, it can be also
deleted

o device replace - ditto

When there are no active swapfiles and a whole-filesystem exclusive
operation is running (e.g. balance, device delete, shrink), the
swapfiles cannot be temporarily activated. The operation must finish
first.

To create and activate a swapfile run the following commands:

# truncate -s 0 swapfile
# chattr +C swapfile
# fallocate -l 2G swapfile
# chmod 0600 swapfile
# mkswap swapfile
# swapon swapfile

Since version 6.1 it's possible to create the swapfile in a single
command (except the activation):

# btrfs filesystem mkswapfile --size 2G swapfile
# swapon swapfile

Please note that the UUID returned by the mkswap utility identifies the
swap "filesystem" and because it's stored in a file, it's not generally
visible and usable as an identifier unlike if it was on a block device.

Once activated the file will appear in /proc/swaps:

# cat /proc/swaps
Filename Type Size Used Priority
/path/swapfile file 2097152 0 -2

The swapfile can be created as one-time operation or, once properly
created, activated on each boot by the swapon -a command (usually
started by the service manager). Add the following entry to /etc/fstab,
assuming the filesystem that provides the /path has been already
mounted at this point. Additional mount options relevant for the
swapfile can be set too (like priority, not the BTRFS mount options).

/path/swapfile none swap defaults 0 0

From now on the subvolume with the active swapfile cannot be
snapshotted until the swapfile is deactivated again by swapoff. Then
the swapfile is a regular file and the subvolume can be snapshotted
again, though this would prevent another activation any swapfile that
has been snapshotted. New swapfiles (not snapshotted) can be created
and activated.

Otherwise, an inactive swapfile does not affect the containing
subvolume. Activation creates a temporary in-memory status and prevents
some file operations, but is not stored permanently.

HIBERNATION
A swapfile can be used for hibernation but it's not straightforward.
Before hibernation a resume offset must be written to file
/sys/power/resume_offset or the kernel command line parameter
resume_offset must be set.

The value is the physical offset on the device. Note that this is not
the same value that filefrag prints as physical offset!

Btrfs filesystem uses mapping between logical and physical addresses
but here the physical can still map to one or more device-specific
physical block addresses. It's the device-specific physical offset that
is suitable as resume offset.

Since version 6.1 there's a command btrfs inspect-internal map-swapfile
that will print the device physical offset and the adjusted value for
/sys/power/resume_offset. Note that the value is divided by page size,
i.e. it's not the offset itself.

# btrfs filesystem mkswapfile swapfile
# btrfs inspect-internal map-swapfile swapfile
Physical start: 811511726080
Resume offset: 198122980

For scripting and convenience the option -r will print just the offset:

# btrfs inspect-internal map-swapfile -r swapfile
198122980

The command map-swapfile also verifies all the requirements, i.e. no
holes, single device, etc.

TROUBLESHOOTING
If the swapfile activation fails please verify that you followed all
the steps above or check the system log (e.g. dmesg or journalctl) for
more information.

Notably, the swapon utility exits with a message that does not say what
failed:

# swapon /path/swapfile
swapon: /path/swapfile: swapon failed: Invalid argument

The specific reason is likely to be printed to the system log by the
btrfs module:

# journalctl -t kernel | grep swapfile
kernel: BTRFS warning (device sda): swapfile must have single data profile

CHECKSUM ALGORITHMS
Data and metadata are checksummed by default. The checksum is
calculated before writing and verified after reading the blocks from
devices. The whole metadata block has an inline checksum stored in the
b-tree node header. Each data block has a detached checksum stored in
the checksum tree.

NOTE:
Since a data checksum is calculated just before submitting to the
block device, btrfs has a strong requirement that the corresponding
data block must not be modified until the writeback is finished.

This requirement is met for a buffered write as btrfs has the full
control on its page cache, but a direct write (O_DIRECT) bypasses
page cache, and btrfs can not control the direct IO buffer (as it
can be in user space memory). Thus it's possible that a user space
program modifies its direct write buffer before the buffer is fully
written back, and this can lead to a data checksum mismatch.

To avoid this, kernel starting with version 6.14 will force a direct
write to fall back to buffered, if the inode requires a data
checksum. This will bring a small performance penalty. If you
require true zero-copy direct writes, then set the NODATASUM flag
for the inode and make sure the direct IO buffer is fully aligned to
block size.

There are several checksum algorithms supported. The default and
backward compatible algorithm is crc32c. Since kernel 5.5 there are
three more with different characteristics and trade-offs regarding
speed and strength. The following list may help you to decide which one
to select.

CRC32C (32 bits digest)
Default, best backward compatibility. Very fast, modern CPUs
have instruction-level support, not collision-resistant but
still good error detection capabilities.

XXHASH (64 bits digest)
Can be used as CRC32C successor. Very fast, optimized for modern
CPUs utilizing instruction pipelining, good collision resistance
and error detection.

SHA256 (256 bits digest)
Cryptographic-strength hash. Relatively slow but with possible
CPU instruction acceleration or specialized hardware cards. FIPS
certified and in wide use.

BLAKE2b (256 bits digest)
Cryptographic-strength hash. Relatively fast, with possible CPU
acceleration using SIMD extensions. Not standardized but based
on BLAKE which was a SHA3 finalist, in wide use. The algorithm
used is BLAKE2b-256 that's optimized for 64-bit platforms.

The digest size affects overall size of data block checksums stored in
the filesystem. The metadata blocks have a fixed area up to 256 bits
(32 bytes), so there's no increase. Each data block has a separate
checksum stored, with additional overhead of the b-tree leaves.

Approximate relative performance of the algorithms, measured against
CRC32C using implementations on a 11th gen 3.6GHz intel CPU:

+--------+-------------+-------+------------------+
|Digest | Cycles/4KiB | Ratio | Implementation |
+--------+-------------+-------+------------------+
|CRC32C | 470 | 1.00 | CPU instruction, |
| | | | PCL combination |
+--------+-------------+-------+------------------+
|XXHASH | 870 | 1.9 | reference impl. |
+--------+-------------+-------+------------------+
|SHA256 | 7600 | 16 | libgcrypt |
+--------+-------------+-------+------------------+
|SHA256 | 8500 | 18 | openssl |
+--------+-------------+-------+------------------+
|SHA256 | 8700 | 18 | botan |
+--------+-------------+-------+------------------+
|SHA256 | 32000 | 68 | builtin, CPU |
| | | | instruction |
+--------+-------------+-------+------------------+
|SHA256 | 37000 | 78 | libsodium |
+--------+-------------+-------+------------------+
|SHA256 | 78000 | 166 | builtin, |
| | | | reference impl. |
+--------+-------------+-------+------------------+
|BLAKE2b | 10000 | 21 | builtin/AVX2 |
+--------+-------------+-------+------------------+
|BLAKE2b | 10900 | 23 | libgcrypt |
+--------+-------------+-------+------------------+
|BLAKE2b | 13500 | 29 | builtin/SSE41 |
+--------+-------------+-------+------------------+
|BLAKE2b | 13700 | 29 | libsodium |
+--------+-------------+-------+------------------+
|BLAKE2b | 14100 | 30 | openssl |
+--------+-------------+-------+------------------+
|BLAKE2b | 14500 | 31 | kcapi |
+--------+-------------+-------+------------------+
|BLAKE2b | 14500 | 34 | builtin, |
| | | | reference impl. |
+--------+-------------+-------+------------------+
Many kernels are configured with SHA256 as built-in and not as a
module. The accelerated versions are however provided by the modules
and must be loaded explicitly (modprobe sha256) before mounting the
filesystem to make use of them. You can check in
/sys/fs/btrfs/FSID/checksum which one is used. If you see
sha256-generic, then you may want to unmount and mount the filesystem
again. Changing that on a mounted filesystem is not possible. Check
the file /proc/crypto, when the implementation is built-in, you'd find:

name : sha256
driver : sha256-generic
module : kernel
priority : 100
...

While accelerated implementation is e.g.:

name : sha256
driver : sha256-avx2
module : sha256_ssse3
priority : 170
...

COMPRESSION
Btrfs supports transparent file compression. There are three algorithms
available: ZLIB, LZO and ZSTD (since v4.14), with various levels. The
compression happens on the level of file extents and the algorithm is
selected by file property, mount option or by a defrag command. You
can have a single btrfs mount point that has some files that are
uncompressed, some that are compressed with LZO, some with ZLIB, for
instance (though you may not want it that way, it is supported).

Once the compression is set, all newly written data will be compressed,
i.e. existing data are untouched. Data are split into smaller chunks
(128KiB) before compression to make random rewrites possible without a
high performance hit. Due to the increased number of extents the
metadata consumption is higher. The chunks are compressed in parallel.

The algorithms can be characterized as follows regarding the
speed/ratio trade-offs:

ZLIB

o slower, higher compression ratio

o levels: 1 to 9, mapped directly, default level is 3

o good backward compatibility

LZO

o faster compression and decompression than ZLIB, worse
compression ratio, designed to be fast

o no levels

o good backward compatibility

ZSTD

o compression comparable to ZLIB with higher
compression/decompression speeds and different ratio

o levels: -15..15, mapped directly, default is 3

o support since 4.14

o levels 1..15 supported since 5.1

o levels -15..-1 supported since 6.15

The differences depend on the actual data set and cannot be expressed
by a single number or recommendation. Higher levels consume more CPU
time and may not bring a significant improvement, lower levels are
close to real time.

HOW TO ENABLE COMPRESSION
Typically the compression can be enabled on the whole filesystem,
specified for the mount point. Note that the compression mount options
are shared among all mounts of the same filesystem, either bind mounts
or subvolume mounts. Please refer to btrfs(5) section MOUNT OPTIONS.

$ mount -o compress=zstd /dev/sdx /mnt

This will enable the zstd algorithm on the default level (which is 3).
The level can be specified manually too like zstd:3. Higher levels
compress better at the cost of time. This in turn may cause increased
write latency, low levels are suitable for real-time compression and on
reasonably fast CPU don't cause noticeable performance drops.

$ btrfs filesystem defrag -czstd file

The command above will start defragmentation of the whole file and
apply the compression, regardless of the mount option. (Note:
specifying level is not yet implemented). The compression algorithm is
not persistent and applies only to the defragmentation command, for any
other writes other compression settings apply.

Persistent settings on a per-file basis can be set in two ways:

$ chattr +c file
$ btrfs property set file compression zstd

The first command is using legacy interface of file attributes
inherited from ext2 filesystem and is not flexible, so by default the
zlib compression is set. The other command sets a property on the file
with the given algorithm. (Note: setting level that way is not yet
implemented.)

COMPRESSION LEVELS
The level support of ZLIB has been added in v4.14, LZO does not support
levels (the kernel implementation provides only one), ZSTD level
support has been added in v5.1 and the negative levels in v6.15.

There are 9 levels of ZLIB supported (1 to 9), mapping 1:1 from the
mount option to the algorithm defined level. The default is level 3,
which provides the reasonably good compression ratio and is still
reasonably fast. The difference in compression gain of levels 7, 8 and
9 is comparable but the higher levels take longer.

The ZSTD support includes levels -15..15, a subset of full range of
what ZSTD provides. Levels -15..-1 are real-time with worse compression
ratio, levels 1..3 are near real-time with good compression, 4..8 are
slower with improved compression and 9..15 try even harder though the
resulting size may not be significantly improved. Higher levels also
require more memory and as they need more CPU the system performance is
affected.

Level 0 always maps to the default. The compression level does not
affect compatibility.

INCOMPRESSIBLE DATA
Files with already compressed data or with data that won't compress
well with the CPU and memory constraints of the kernel implementations
are using a simple decision logic. If the first portion of data being
compressed is not smaller than the original, the compression of the
file is disabled -- unless the filesystem is mounted with
compress-force. In that case compression will always be attempted on
the file only to be later discarded. This is not optimal and subject to
optimizations and further development.

If a file is identified as incompressible, a flag is set (NOCOMPRESS)
and it's sticky. On that file compression won't be performed unless
forced. The flag can be also set by chattr +m (since e2fsprogs 1.46.2)
or by properties with value no or none. Empty value will reset it to
the default that's currently applicable on the mounted filesystem.

There are two ways to detect incompressible data:

o actual compression attempt - data are compressed, if the result is
not smaller, it's discarded, so this depends on the algorithm and
level

o pre-compression heuristics - a quick statistical evaluation on the
data is performed and based on the result either compression is
performed or skipped, the NOCOMPRESS bit is not set just by the
heuristic, only if the compression algorithm does not make an
improvement

$ lsattr file
---------------------m file

Using the forcing compression is not recommended, the heuristics are
supposed to decide that and compression algorithms internally detect
incompressible data too.

PRE-COMPRESSION HEURISTICS
The heuristics aim to do a few quick statistical tests on the
compressed data in order to avoid probably costly compression that
would turn out to be inefficient. Compression algorithms could have
internal detection of incompressible data too but this leads to more
overhead as the compression is done in another thread and has to write
the data anyway. The heuristic is read-only and can utilize cached
memory.

The tests performed based on the following: data sampling, long
repeated pattern detection, byte frequency, Shannon entropy.

COMPATIBILITY
Compression is done using the COW mechanism so it's incompatible with
nodatacow. Direct IO read works on compressed files but will fall back
to buffered writes and leads to no compression even if force
compression is set. Currently nodatasum and compression don't work
together.

The compression algorithms have been added over time so the version
compatibility should be also considered, together with other tools that
may access the compressed data like bootloaders.

SYSFS INTERFACE
Btrfs has a sysfs interface to provide extra knobs.

The top level path is /sys/fs/btrfs/, and the main directory layout is
the following:

For /sys/fs/btrfs/<UUID>/features/ directory, each file means an
enabled feature on the mounted filesystem.

The features share the same name in section FILESYSTEM FEATURES.

UUID
Files in /sys/fs/btrfs/<UUID>/ directory are:

bg_reclaim_threshold
(RW, since: 5.19)

Used space percentage of total device space to start auto block
group claim. Mostly for zoned devices.

checksum
(RO, since: 5.5)

The checksum used for the mounted filesystem. This includes
both the checksum type (see section CHECKSUM ALGORITHMS) and the
implemented driver (mostly shows if it's hardware accelerated).

clone_alignment
(RO, since: 3.16)

The bytes alignment for clone and dedupe ioctls.

commit_stats
(RW, since: 6.0)

The performance statistics for btrfs transaction commit since
the first mount. Mostly for debugging purposes.

Writing into this file will reset the maximum commit duration
(max_commit_ms) to 0. The file looks like:

commits 70649
last_commit_ms 2
max_commit_ms 131
total_commit_ms 170840

o commits - number of transaction commits since the first mount

o last_commit_ms - duration in milliseconds of the last commit

o max_commit_ms - maximum time a transaction commit took since
first mount or last reset

o total_commit_ms - sum of all transaction commit times

exclusive_operation
(RO, since: 5.10)

Shows the running exclusive operation. Check section FILESYSTEM
EXCLUSIVE OPERATIONS for details.

generation
(RO, since: 5.11)

Show the generation of the mounted filesystem.

label (RW, since: 3.14)

Show the current label of the mounted filesystem.

metadata_uuid
(RO, since: 5.0)

Shows the metadata UUID of the mounted filesystem. Check
metadata_uuid feature for more details.

nodesize
(RO, since: 3.14)

Show the nodesize of the mounted filesystem.

quota_override
(RW, since: 4.13)

Shows the current quota override status. 0 means no quota
override. 1 means quota override, quota can ignore the existing
limit settings.

read_policy
(RW, since: 5.11)

Shows the current balance policy for reads. Currently only pid
(balance using the process id (pid) value) is supported. More
balancing policies are available in experimental build, namely
round-robin.

sectorsize
(RO, since: 3.14)

Shows the sectorsize of the mounted filesystem.

temp_fsid
(RO, since 6.7)

Indicate that this filesystem got assigned a temporary FSID at
mount time, making possible to mount devices with the same FSID.

UUID/allocations
Files and directories in /sys/fs/btrfs/<UUID>/allocations directory
are:

global_rsv_reserved
(RO, since: 3.14)

The used bytes of the global reservation.

global_rsv_size
(RO, since: 3.14)

The total size of the global reservation.

data/, metadata/ and system/ directories
(RO, since: 5.14)

Space info accounting for the 3 block group types.

UUID/allocations/{data,metadata,system}
Files in /sys/fs/btrfs/<UUID>/allocations/data,metadata,system
directory are:

bg_reclaim_threshold
(RW, since: 5.19)

Reclaimable space percentage of block group's size (excluding
permanently unusable space) to reclaim the block group. Can be
used on regular or zoned devices.

bytes_*
(RO)

Values of the corresponding data structures for the given block
group type and profile that are used internally and may change
rapidly depending on the load.

Complete list: bytes_may_use, bytes_pinned, bytes_readonly,
bytes_reserved, bytes_used, bytes_zone_unusable

chunk_size
(RW, since: 6.0)

Shows the chunk size. Can be changed for data and metadata
(independently) and cannot be set for system block group type.
Cannot be set for zoned devices as it depends on the fixed
device zone size. Upper bound is 10% of the filesystem size,
the value must be multiple of 256MiB and greater than 0.

size_classes
(RO, since: 6.3)

Numbers of block groups of a given classes based on heuristics
that measure extent length, age and fragmentation.

none 136
small 374
medium 282
large 93

UUID/bdi
Symlink to the sysfs directory of the backing device info (BDI), which
is related to writeback process and infrastructure.

UUID/devices
Files in /sys/fs/btrfs/<UUID>/devices directory are symlinks named
after device nodes (e.g. sda, dm-0) and pointing to their sysfs
directory.

UUID/devinfo
The directory contains subdirectories named after device ids (numeric
values). Each subdirectory has information about the device of the
given devid.

UUID/devinfo/DEVID
Files in /sys/fs/btrfs/<UUID>/devinfo/<DEVID> directory are:

error_stats:
(RO, since: 5.14)

Shows device stats of this device, same as btrfs device stats
(btrfs-device(8)).

write_errs 0
read_errs 0
flush_errs 0
corruption_errs 0
generation_errs 0

fsid: (RO, since: 5.17)

Shows the fsid which the device belongs to. It can be different
than the UUID if it's a seed device.

in_fs_metadata
(RO, since: 5.6)

Shows whether we have found the device. Should always be 1, as
if this turns to 0, the DEVID directory would get removed
automatically.

missing
(RO, since: 5.6)

Shows whether the device is considered missing by the kernel
module.

replace_target
(RO, since: 5.6)

Shows whether the device is the replace target. If no device
replace is running, this value is 0.

scrub_speed_max
(RW, since: 5.14)

Shows the scrub speed limit for this device. The unit is
Bytes/s. 0 means no limit. The value can be set but is not
persistent.

writeable
(RO, since: 5.6)

Show if the device is writeable.

UUID/qgroups
Files in /sys/fs/btrfs/<UUID>/qgroups/ directory are:

enabled
(RO, since: 6.1)

Shows if qgroup is enabled. Also, if qgroup is disabled, the
qgroups directory will be removed automatically.

inconsistent
(RO, since: 6.1)

Shows if the qgroup numbers are inconsistent. If 1, it's
recommended to do a qgroup rescan.

drop_subtree_threshold
(RW, since: 6.1)

Shows the subtree drop threshold to automatically mark qgroup
inconsistent.

When dropping large subvolumes with qgroup enabled, there would
be a huge load for qgroup accounting. If we have a subtree
whose level is larger than or equal to this value, we will not
trigger qgroup account at all, but mark qgroup inconsistent to
avoid the huge workload.

Default value is 3, which means that trees of low height will be
accounted properly as this is sufficiently fast. The value was 8
until 6.13 where no subtree drop can trigger qgroup rescan
making it less useful.

Lower value can reduce qgroup workload, at the cost of extra
qgroup rescan to re-calculate the numbers.

UUID/qgroups/LEVEL_ID
Files in each /sys/fs/btrfs/<UUID>/qgroups/<LEVEL>_<ID>/ directory are:

exclusive
(RO, since: 5.9)

Shows the exclusively owned bytes of the qgroup.

limit_flags
(RO, since: 5.9)

Shows the numeric value of the limit flags. If 0, means no
limit implied.

max_exclusive
(RO, since: 5.9)

Shows the limits on exclusively owned bytes.

max_referenced
(RO, since: 5.9)

Shows the limits on referenced bytes.

referenced
(RO, since: 5.9)

Shows the referenced bytes of the qgroup.

rsv_data
(RO, since: 5.9)

Shows the reserved bytes for data.

rsv_meta_pertrans
(RO, since: 5.9)

Shows the reserved bytes for per transaction metadata.

rsv_meta_prealloc
(RO, since: 5.9)

Shows the reserved bytes for preallocated metadata.

UUID/discard
Files in /sys/fs/btrfs/<UUID>/discard/ directory are:

discardable_bytes
(RO, since: 6.1)

Shows amount of bytes that can be discarded in the async discard
and nodiscard mode.

discardable_extents
(RO, since: 6.1)

Shows number of extents to be discarded in the async discard and
nodiscard mode.

discard_bitmap_bytes
(RO, since: 6.1)

Shows amount of discarded bytes from data tracked as bitmaps.

discard_extent_bytes
(RO, since: 6.1)

Shows amount of discarded extents from data tracked as bitmaps.

discard_bytes_saved
(RO, since: 6.1)

Shows the amount of bytes that were reallocated without being
discarded.

kbps_limit
(RW, since: 6.1)

Tunable limit of kilobytes per second issued as discard IO in
the async discard mode.

iops_limit
(RW, since: 6.1)

Tunable limit of number of discard IO operations to be issued in
the async discard mode.

max_discard_size
(RW, since: 6.1)

Tunable limit for size of one IO discard request.

FILESYSTEM EXCLUSIVE OPERATIONS
There are several operations that affect the whole filesystem and
cannot be run in parallel. Attempt to start one while another is
running will fail (see exceptions below).

Since kernel 5.10 the currently running operation can be obtained from
/sys/fs/UUID/exclusive_operation with following values and operations:

o balance

o balance paused (since 5.17)

o device add

o device delete

o device replace

o resize

o swapfile activate

o none

Enqueuing is supported for several btrfs subcommands so they can be
started at once and then serialized.

There's an exception when a paused balance allows to start a device add
operation as they don't really collide and this can be used to add more
space for the balance to finish.

FILESYSTEM LIMITS

maximum file name length
255

This limit is imposed by Linux VFS, the structures of BTRFS
could store larger file names.

maximum symlink target length
depends on the nodesize value, for 4KiB it's 3949 bytes, for
larger nodesize it's 4095 due to the system limit PATH_MAX

The symlink target may not be a valid path, i.e. the path name
components can exceed the limits (NAME_MAX), there's no content
validation at symlink(3) creation.

maximum number of inodes
264 but depends on the available metadata space as the inodes
are created dynamically

Each subvolume is an independent namespace of inodes and thus
their numbers, so the limit is per subvolume, not for the whole
filesystem.

inode numbers
minimum number: 256 (for subvolumes), regular files and
directories: 257, maximum number: (264 - 256)

The inode numbers that can be assigned to user created files are
from the whole 64bit space except first 256 and last 256 in that
range that are reserved for internal b-tree identifiers.

maximum file length
inherent limit of BTRFS is 264 (16 EiB) but the practical limit
of Linux VFS is 263 (8 EiB)

maximum number of subvolumes
the subvolume ids can go up to 248 but the number of actual
subvolumes depends on the available metadata space

The space consumed by all subvolume metadata includes
bookkeeping of shared extents can be large (MiB, GiB). The range
is not the full 64bit range because of qgroups that use the
upper 16 bits for another purposes.

maximum number of hardlinks of a file in a directory
65536 when the extref feature is turned on during mkfs
(default), roughly 100 otherwise and depends on file name length
that fits into one metadata node

minimum filesystem size
the minimal size of each device depends on the mixed-bg feature,
without that (the default) it's about 109MiB, with mixed-bg it's
is 16MiB

BOOTLOADER SUPPORT
GRUB2 (https://www.gnu.org/software/grub) has the most advanced support
of booting from BTRFS with respect to features.

U-Boot (https://www.denx.de/wiki/U-Boot/) has decent support for
booting but not all BTRFS features are implemented, check the
documentation.

In general, the first 1MiB on each device is unused with the exception
of primary superblock that is on the offset 64KiB and spans 4KiB. The
rest can be freely used by bootloaders or for other system information.
Note that booting from a filesystem on zoned device is not supported.

FILE ATTRIBUTES
The btrfs filesystem supports setting file attributes or flags. Note
there are old and new interfaces, with confusing names. The following
list should clarify that:

o attributes: chattr(1) or lsattr(1) utilities (the ioctls are
FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the
attributes are also called flags

o xflags: to distinguish from the previous, it's extended flags, with
tunable bits similar to the attributes but extensible and new bits
will be added in the future (the ioctls are FS_IOC_FSGETXATTR and
FS_IOC_FSSETXATTR but they are not related to extended attributes
that are also called xattrs), there's no standard tool to change the
bits, there's support in xfs_io(8) as command xfs_io -c chattr

Attributes

a append only, new writes are always written at the end of the
file

A no atime updates

c compress data, all data written after this attribute is set will
be compressed. Please note that compression is also affected by
the mount options or the parent directory attributes.

When set on a directory, all newly created files will inherit
this attribute. This attribute cannot be set with 'm' at the
same time.

C no copy-on-write, file data modifications are done in-place

When set on a directory, all newly created files will inherit
this attribute.

NOTE:
Due to implementation limitations, this flag can be set/unset
only on empty files.

d no dump, makes sense with 3rd party tools like dump(8), on BTRFS
the attribute can be set/unset but no other special handling is
done

D synchronous directory updates, for more details search open(2)
for O_SYNC and O_DSYNC

i immutable, no file data and metadata changes allowed even to the
root user as long as this attribute is set (obviously the
exception is unsetting the attribute)

m no compression, permanently turn off compression on the given
file. Any compression mount options will not affect this file.
(chattr(1) support added in 1.46.2)

When set on a directory, all newly created files will inherit
this attribute. This attribute cannot be set with c at the same
time.

S synchronous updates, for more details search open(2) for O_SYNC
and O_DSYNC

No other attributes are supported. For the complete list please refer
to the chattr(1) manual page.

XFLAGS
There's an overlap of letters assigned to the bits with the attributes,
this list refers to what xfs_io(8) provides:

i immutable, same as the attribute

a append only, same as the attribute

s synchronous updates, same as the attribute S

A no atime updates, same as the attribute

d no dump, same as the attribute

ZONED MODE
Since version 5.12 btrfs supports so called zoned mode. This is a
special on-disk format and allocation/write strategy that's friendly to
zoned devices. In short, a device is partitioned into fixed-size zones
and each zone can be updated by append-only manner, or reset. As btrfs
has no fixed data structures, except the super blocks, the zoned mode
only requires block placement that follows the device constraints. You
can learn about the whole architecture at https://zonedstorage.io .

The devices are also called SMR/ZBC/ZNS, in host-managed mode. Note
that there are devices that appear as non-zoned but actually are, this
is drive-managed and using zoned mode won't help.

The zone size depends on the device, typical sizes are 256MiB or 1GiB.
In general it must be a power of two. Emulated zoned devices like
null_blk allow to set various zone sizes.

Requirements, limitations

o all devices must have the same zone size

o maximum zone size is 8GiB

o minimum zone size is 4MiB

o mixing zoned and non-zoned devices is possible, the zone writes are
emulated, but this is namely for testing

o the super block is handled in a special way and is at different
locations than on a non-zoned filesystem:

o primary: 0B (and the next two zones)

o secondary: 512GiB (and the next two zones)

o tertiary: 4TiB (4096GiB, and the next two zones)

Incompatible features
The main constraint of the zoned devices is lack of in-place update of
the data. This is inherently incompatible with some features:

o NODATACOW - overwrite in-place, cannot create such files

o fallocate - preallocating space for in-place first write

o mixed-bg - unordered writes to data and metadata, fixing that means
using separate data and metadata block groups

o booting - the zone at offset 0 contains superblock, resetting the
zone would destroy the bootloader data

Initial support lacks some features but they're planned:

o only single (data, metadata) and DUP (metadata) profile is supported

o fstrim - due to dependency on free space cache v1

Super block
As said above, super block is handled in a special way. In order to be
crash safe, at least one zone in a known location must contain a valid
superblock. This is implemented as a ring buffer in two consecutive
zones, starting from known offsets 0B, 512GiB and 4TiB.

The values are different than on non-zoned devices. Each new super
block is appended to the end of the zone, once it's filled, the zone is
reset and writes continue to the next one. Looking up the latest super
block needs to read offsets of both zones and determine the last
written version.

The amount of space reserved for super block depends on the zone size.
The secondary and tertiary copies are at distant offsets as the
capacity of the devices is expected to be large, tens of terabytes.
Maximum zone size supported is 8GiB, which would mean that e.g. offset
0-16GiB would be reserved just for the super block on a hypothetical
device of that zone size. This is wasteful but required to guarantee
crash safety.

Zone reclaim, garbage collection
As the zones are append-only, overwriting data or COW changes in
metadata make parts of the zones used but not connected to the
filesystem structures. This makes the space unusable and grows over
time. Once the ratio hits a (configurable) threshold a background
reclaim process is started and relocates the remaining blocks in use to
a new zone. The old one is reset and can be used again.

This process may take some time depending on other background work or
amount of new data written. It is possible to hit an intermittent
ENOSPC. Some devices also limit number of active zones.

Devices
Real hardware
The WD Ultrastar series 600 advertises HM-SMR, i.e. the host-managed
zoned mode. There are two more: DA (device managed, no zoned
information exported to the system), HA (host aware, can be used as
regular disk but zoned writes improve performance). There are not many
devices available at the moment, the information about exact zoned mode
is hard to find, check data sheets or community sources gathering
information from real devices.

Note: zoned mode won't work with DM-SMR disks.

o Ultrastar(R) DC ZN540 NVMe ZNS SSD (product brief)

Emulated: null_blk
The driver null_blk provides memory backed device and is suitable for
testing. There are some quirks setting up the devices. The module must
be loaded with nr_devices=0 or the numbering of device nodes will be
offset. The configfs must be mounted at /sys/kernel/config and the
administration of the null_blk devices is done in
/sys/kernel/config/nullb. The device nodes are named like /dev/nullb0
and are numbered sequentially. NOTE: the device name may be different
than the named directory in sysfs!

Setup:

modprobe configfs
modprobe null_blk nr_devices=0

Create a device mydev, assuming no other previously created devices,
size is 2048MiB, zone size 256MiB. There are more tunable parameters,
this is a minimal example taking defaults:

cd /sys/kernel/config/nullb/
mkdir mydev
cd mydev
echo 2048 > size
echo 1 > zoned
echo 1 > memory_backed
echo 256 > zone_size
echo 1 > power

This will create a device /dev/nullb0 and the value of file index will
match the ending number of the device node.

Remove the device:

rmdir /sys/kernel/config/nullb/mydev

Then continue with mkfs.btrfs /dev/nullb0, the zoned mode is
auto-detected.

For convenience, there's a script wrapping the basic null_blk
management operations https://github.com/kdave/nullb.git, the above
commands become:

nullb setup
nullb create -s 2g -z 256
mkfs.btrfs /dev/nullb0
...
nullb rm nullb0

Emulated: TCMU runner
TCMU is a framework to emulate SCSI devices in userspace, providing
various backends for the storage, with zoned support as well. A
file-backed zoned device can provide more options for larger storage
and zone size. Please follow the instructions at
https://zonedstorage.io/projects/tcmu-runner/ .

Compatibility, incompatibility

o the feature sets an incompat bit and requires new kernel to access
the filesystem (for both read and write)

o superblock needs to be handled in a special way, there are still 3
copies but at different offsets (0, 512GiB, 4TiB) and the 2
consecutive zones are a ring buffer of the superblocks, finding the
latest one needs reading it from the write pointer or do a full scan
of the zones

o mixing zoned and non zoned devices is possible (zones are emulated)
but is recommended only for testing

o mixing zoned devices with different zone sizes is not possible

o zone sizes must be power of two, zone sizes of real devices are e.g.
256MiB or 1GiB, larger size is expected, maximum zone size supported
by btrfs is 8GiB

Status, stability, reporting bugs
The zoned mode has been released in 5.12 and there are still some rough
edges and corner cases one can hit during testing. Please report bugs
to https://github.com/naota/linux/issues/ .

References

o https://zonedstorage.io

o https://zonedstorage.io/projects/libzbc/ -- libzbc is library and
set of tools to directly manipulate devices with ZBC/ZAC support

o https://zonedstorage.io/projects/libzbd/ -- libzbd uses the kernel
provided zoned block device interface based on the ioctl() system
calls

o https://hddscan.com/blog/2020/hdd-wd-smr.html -- some details about
exact device types

o https://lwn.net/Articles/853308/ -- Btrfs on zoned block devices

o https://www.usenix.org/conference/vault20/presentation/bjorling --
Zone Append: A New Way of Writing to Zoned Storage

CONTROL DEVICE
There's a character special device /dev/btrfs-control with major and
minor numbers 10 and 234 (the device can be found under the misc
category).

$ ls -l /dev/btrfs-control
crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control

The device accepts some ioctl calls that can perform following actions
on the filesystem module:

o scan devices for btrfs filesystem (i.e. to let multi-device
filesystems mount automatically) and register them with the kernel
module

o similar to scan, but also wait until the device scanning process is
finished for a given filesystem

o get the supported features (can be also found under
/sys/fs/btrfs/features)

The device is created when btrfs is initialized, either as a module or
a built-in functionality and makes sense only in connection with that.
Running e.g. mkfs without the module loaded will not register the
device and will probably warn about that.

In rare cases when the module is loaded but the device is not present
(most likely accidentally deleted), it's possible to recreate it by

# mknod --mode=600 /dev/btrfs-control c 10 234

or (since 5.11) by a convenience command

# btrfs rescue create-control-device

The control device is not strictly required but the device scanning
will not work and a workaround would need to be used to mount a
multi-device filesystem. The mount option device can trigger the
device scanning during mount, see also btrfs device scan.

FILESYSTEM WITH MULTIPLE PROFILES
It is possible that a btrfs filesystem contains multiple block group
profiles of the same type. This could happen when a profile conversion
using balance filters is interrupted (see btrfs-balance(8)). Some
btrfs commands perform a test to detect this kind of condition and
print a warning like this:

WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1

The corresponding output of btrfs filesystem df might look like:

WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
Data, RAID1: total=832.00MiB, used=0.00B
Data, single: total=1.63GiB, used=0.00B
System, single: total=4.00MiB, used=16.00KiB
Metadata, single: total=8.00MiB, used=112.00KiB
Metadata, RAID1: total=64.00MiB, used=32.00KiB
GlobalReserve, single: total=16.25MiB, used=0.00B

There's more than one line for type Data and Metadata, while the
profiles are single and RAID1.

This state of the filesystem OK but most likely needs the
user/administrator to take an action and finish the interrupted tasks.
This cannot be easily done automatically, also the user knows the
expected final profiles.

In the example above, the filesystem started as a single device and
single block group profile. Then another device was added, followed by
balance with convert=raid1 but for some reason hasn't finished.
Restarting the balance with convert=raid1 will continue and end up with
filesystem with all block group profiles RAID1.

NOTE:
If you're familiar with balance filters, you can use
convert=raid1,profiles=single,soft, which will take only the
unconverted single profiles and convert them to raid1. This may
speed up the conversion as it would not try to rewrite the already
convert raid1 profiles.

Having just one profile is desired as this also clearly defines the
profile of newly allocated block groups, otherwise this depends on
internal allocation policy. When there are multiple profiles present,
the order of selection is RAID56, RAID10, RAID1, RAID0 as long as the
device number constraints are satisfied.

Commands that print the warning were chosen so they're brought to user
attention when the filesystem state is being changed in that regard.
This is: device add, device delete, balance cancel, balance pause.
Commands that report space usage: filesystem df, device usage. The
command filesystem usage provides a line in the overall summary:

Multiple profiles: yes (data, metadata)

SEEDING DEVICE
The COW mechanism and multiple devices under one hood enable an
interesting concept, called a seeding device: extending a read-only
filesystem on a device with another device that captures all writes.
For example imagine an immutable golden image of an operating system
enhanced with another device that allows to use the data from the
golden image and normal operation. This idea originated on CD-ROMs
with base OS and allowing to use them for live systems, but this became
obsolete. There are technologies providing similar functionality, like
unionmount, overlayfs or qcow2 image snapshot.

The seeding device starts as a normal filesystem, once the contents is
ready, btrfstune -S 1 is used to flag it as a seeding device. Mounting
such device will not allow any writes, except adding a new device by
btrfs device add. Then the filesystem can be remounted as read-write.

Given that the filesystem on the seeding device is always recognized as
read-only, it can be used to seed multiple filesystems from one device
at the same time. The UUID that is normally attached to a device is
automatically changed to a random UUID on each mount.

Once the seeding device is mounted, it needs the writable device. After
adding it, unmounting and mounting with umount /path; mount
/dev/writable /path or remounting read-write with remount -o remount,rw
makes the filesystem at /path ready for use.

NOTE:
There is a known bug with using remount to make the mount writeable:
remount will leave the filesystem in a state where it is unable to
clean deleted snapshots, so it will leak space until it is unmounted
and mounted properly.

Furthermore, deleting the seeding device from the filesystem can turn
it into a normal filesystem, provided that the writable device can also
contain all the data from the seeding device.

The seeding device flag can be cleared again by btrfstune -f -S 0, e.g.
allowing to update with newer data but please note that this will
invalidate all existing filesystems that use this particular seeding
device. This works for some use cases, not for others, and the forcing
flag to the command is mandatory to avoid accidental mistakes.

Example how to create and use one seeding device:

# mkfs.btrfs /dev/sda
# mount /dev/sda /mnt/mnt1
... fill mnt1 with data
# umount /mnt/mnt1

# btrfstune -S 1 /dev/sda

# mount /dev/sda /mnt/mnt1
# btrfs device add /dev/sdb /mnt/mnt1
# umount /mnt/mnt1
# mount /dev/sdb /mnt/mnt1
... /mnt/mnt1 is now writable

Now /mnt/mnt1 can be used normally. The device /dev/sda can be mounted
again with a another writable device:

# mount /dev/sda /mnt/mnt2
# btrfs device add /dev/sdc /mnt/mnt2
# umount /mnt/mnt2
# mount /dev/sdc /mnt/mnt2
... /mnt/mnt2 is now writable

The writable device (file:/dev/sdb) can be decoupled from the seeding
device and used independently:

# btrfs device delete /dev/sda /mnt/mnt1

As the contents originated in the seeding device, it's possible to turn
/dev/sdb to a seeding device again and repeat the whole process.

A few things to note:

o it's recommended to use only single device for the seeding device, it
works for multiple devices but the single profile must be used in
order to make the seeding device deletion work

o block group profiles single and dup support the use cases above

o the label is copied from the seeding device and can be changed by
btrfs filesystem label

o each new mount of the seeding device gets a new random UUID

o umount /path; mount /dev/writable /path can be replaced with mount -o
remount,rw /path but it won't reclaim space of deleted subvolumes
until the seeding device is mounted read-write again before making it
seeding again

Chained seeding devices
Though it's not recommended and is rather an obscure and untested use
case, chaining seeding devices is possible. In the first example, the
writable device /dev/sdb can be turned onto another seeding device
again, depending on the unchanged seeding device /dev/sda. Then using
/dev/sdb as the primary seeding device it can be extended with another
writable device, say /dev/sdd, and it continues as before as a simple
tree structure on devices.

# mkfs.btrfs /dev/sda
# mount /dev/sda /mnt/mnt1
... fill mnt1 with data
# umount /mnt/mnt1

# btrfstune -S 1 /dev/sda

# mount /dev/sda /mnt/mnt1
# btrfs device add /dev/sdb /mnt/mnt1
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
# umount /mnt/mnt1

# btrfstune -S 1 /dev/sdb

# mount /dev/sdb /mnt/mnt1
# btrfs device add /dev/sdc /mnt
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
# umount /mnt/mnt1

As a result we have:

o sda is a single seeding device, with its initial contents

o sdb is a seeding device but requires sda, the contents are from the
time when sdb is made seeding, i.e. contents of sda with any later
changes

o sdc last writable, can be made a seeding one the same way as was sdb,
preserving its contents and depending on sda and sdb

As long as the seeding devices are unmodified and available, they can
be used to start another branch.

RAID56 STATUS AND RECOMMENDED PRACTICES
The RAID56 feature provides striping and parity over several devices,
same as the traditional RAID5/6. There are some implementation and
design deficiencies that make it unreliable for some corner cases and
the feature should not be used in production, only for evaluation or
testing. The power failure safety for metadata with RAID56 is not
100%.

Metadata
Do not use raid5 nor raid6 for metadata. Use raid1 or raid1c3
respectively.

The substitute profiles provide the same guarantees against loss of 1
or 2 devices, and in some respect can be an improvement. Recovering
from one missing device will only need to access the remaining 1st or
2nd copy, that in general may be stored on some other devices due to
the way RAID1 works on btrfs, unlike on a striped profile (similar to
raid0) that would need all devices all the time.

The space allocation pattern and consumption is different (e.g. on N
devices): for raid5 as an example, a 1GiB chunk is reserved on each
device, while with raid1 there's each 1GiB chunk stored on 2 devices.
The consumption of each 1GiB of used metadata is then N * 1GiB for vs 2
* 1GiB. Using raid1 is also more convenient for balancing/converting to
other profile due to lower requirement on the available chunk space.

Missing/incomplete support
When RAID56 is on the same filesystem with different raid profiles, the
space reporting is inaccurate, e.g. df, btrfs filesystem df or btrfs
filesystem usage. When there's only a one profile per block group type
(e.g. RAID5 for data) the reporting is accurate.

When scrub is started on a RAID56 filesystem, it's started on all
devices that degrade the performance. The workaround is to start it on
each device separately. Due to that the device stats may not match the
actual state and some errors might get reported multiple times.

The write hole problem. An unclean shutdown could leave a partially
written stripe in a state where the some stripe ranges and the parity
are from the old writes and some are new. The information which is
which is not tracked. Write journal is not implemented. Alternatively a
full read-modify-write would make sure that a full stripe is always
written, avoiding the write hole completely, but performance in that
case turned out to be too bad for use.

The striping happens on all available devices (at the time the chunks
were allocated), so in case a new device is added it may not be
utilized immediately and would require a rebalance. A fixed configured
stripe width is not implemented.

GLOSSARY
Terms in italics also appear in this glossary.

allocator
Usually allocator means the block allocator, i.e. the logic
inside the filesystem which decides where to place newly
allocated blocks in order to maintain several constraints (like
data locality, low fragmentation).

In btrfs, allocator may also refer to chunk allocator, i.e. the
logic behind placing chunks on devices.

balance
An operation that can be done to a btrfs filesystem, for example
through btrfs balance /path. A balance passes all data in the
filesystem through the allocator again. It is primarily intended
to rebalance the data in the filesystem across the devices when
a device is added or removed. A balance will regenerate missing
copies for the redundant RAID levels, if a device has failed. As
of Linux kernel 3.3, a balance operation can be made selective
about which parts of the filesystem are rewritten using filters.

barrier
An instruction to the underlying hardware to ensure that
everything before the barrier is physically written to permanent
storage before anything after it. Used in btrfs's copy on write
approach to ensure filesystem consistency.

block A single physically and logically contiguous piece of storage on
a device, of size e.g. 4K. In some contexts also referred to as
sector, though the term block is preferred.

block group
The unit of allocation of space in btrfs. A block group is laid
out on the disk by the btrfs allocator, and will consist of one
or more chunks, each stored on a different device. The number of
chunks used in a block group will depend on its RAID level.

B-tree The fundamental storage data structure used in btrfs. Except for
the superblocks, all of btrfs metadata is stored in one of
several B-trees on disk. B-trees store key/item pairs. While the
same code is used to implement all of the B-trees, there are a
few different categories of B-tree. The name btrfs refers to its
use of B-trees.

btrfsck, fsck, btrfs-check
Tool in btrfs-progs that checks an unmounted filesystem
(offline) and reports on any errors in the filesystem structures
it finds. By default the tool runs in read-only mode as fixing
errors is potentially dangerous. See also scrub.

btrfs-progs
User mode tools to manage btrfs-specific features. Maintained at
http://github.com/kdave/btrfs-progs.git . The main frontend to
btrfs features is the standalone tool btrfs, although other
tools such as mkfs.btrfs and btrfstune are also part of
btrfs-progs.

chunk A part of a block group. Chunks are either 1 GiB in size (for
data) or 256 MiB (for metadata), depending on the overall
filesystem size.

chunk tree
A layer that keeps information about mapping between physical
and logical block addresses. It's stored within the system
group.

cleaner
Usually referred to in context of deleted subvolumes. It's a
background process that removes the actual data once a subvolume
has been deleted. Cleaning can involve lots of IO and CPU
activity depending on the fragmentation and amount of shared
data with other subvolumes.

The cleaner kernel thread also processes defragmentation
triggered by the autodefrag mount option, removing of empty
blocks groups and some other finalization tasks.

copy-on-write, COW
Also known as COW. The method that btrfs uses for modifying
data. Instead of directly overwriting data in place, btrfs
takes a copy of the data, alters it, and then writes the
modified data back to a different (unused) location on the disk.
It then updates the metadata to reflect the new location of the
data. In order to update the metadata, the affected metadata
blocks are also treated in the same way. In COW filesystems,
files tend to fragment as they are modified. Copy-on-write is
also used in the implementation of snapshots and reflink copies.
A copy-on-write filesystem is, in theory, always consistent,
provided the underlying hardware supports barriers.

default subvolume
The subvolume in a btrfs filesystem which is mounted when
mounting the filesystem without using the subvol= mount option.

device A Linux block device, e.g. a whole disk, partition, LVM logical
volume, loopback device, or network block device. A btrfs
filesystem can reside on one or more devices.

df A standard Unix tool for reporting the amount of space used and
free in a filesystem. The standard tool does not give accurate
results, but the btrfs command from btrfs-progs has an
implementation of df which shows space available in more detail.
See the
[[FAQ#Why_does_df_show_incorrect_free_space_for_my_RAID_volume.3F|FAQ]]
for a more detailed explanation of btrfs free space accounting.

DUP A form of "RAID" which stores two copies of each piece of data
on the same device. This is similar to RAID1, and protects
against block-level errors on the device, but does not provide
any guarantees if the entire device fails. By default, btrfs
uses DUP profile for metadata on single device filesystem.s

ENOSPC Error code returned by the OS to a user program when the
filesystem cannot allocate enough data to fulfill the user
request. In most filesystems, it indicates there is no free
space available in the filesystem. Due to the additional space
requirements from btrfs's COW behaviour, btrfs can sometimes
return ENOSPC when there is apparently (in terms of df) a large
amount of space free. This is effectively a bug in btrfs, and
(if it is repeatable), using the mount option enospc_debug may
give a report that will help the btrfs developers. See the
[[FAQ#if_your_device_is_large_.28.3E16GiB.29|FAQ entry]] on free
space.

extent Contiguous sequence of bytes on disk that holds file data. It's
a compact representation that tracks the start and length of the
byte range, so the logic behind allocating blocks (delayed
allocation) strives for maximizing the length before writing the
extents to the devices.

extent buffer
An abstraction of a b-tree metadata block storing item keys and
item data. The underlying related structures are physical device
block and a CPU memory page.

fallocate
Command line tool in util-linux, and a syscall, that reserves
space in the filesystem for a file, without actually writing any
file data to the filesystem. First data write will turn the
preallocated extents into regular ones. See fallocate(1) and
fallocate(2) manual pages for more details.

filefrag
A tool to show the number of extents in a file, and hence the
amount of fragmentation in the file. It is usually part of the
e2fsprogs package on most Linux distributions. While initially
developed for the ext2 filesystem, it works on Btrfs as well. It
uses the FIEMAP ioctl.

free space cache
Also known as "space cache v1". A separate cache tracking free
space as btrfs only tracks the allocated space. The free space
is by definition any hole between allocated ranges. Finding the
free ranges can be I/O intensive so the cache stores a condensed
representation of it. It is updated every transaction commit.

The v1 free space cache has been superseded by free space tree.

free space tree
Successor of free space cache, also known as "space cache v2"
and now default. The free space is tracked in a better way and
using COW unlike a custom mechanism of v1.

fsync On Unix and Unix-like operating systems (of which Linux is the
latter), the fsync(2) system call causes all buffered file
descriptor related data changes to be flushed to the underlying
block device. When a file is modified on a modern operating
system the changes are generally not written to the disk
immediately but rather those changes are buffered in memory for
performance reasons, calling fsync(2) causes any in-memory
changes to be written to disk.

generation
An internal counter which updates for each transaction. When a
metadata block is written (using copy on write), current
generation is stored in the block, so that blocks which are too
new (and hence possibly inconsistent) can be identified.

key A fixed sized tuple used to identify and sort items in a B-tree.
The key is broken up into 3 parts: objectid, type, and offset.
The type field indicates how each of the other two fields should
be used, and what to expect to find in the item.

item A variable sized structure stored in B-tree leaves. Items hold
different types of data depending on key type.

log tree
A b-tree that temporarily tracks ongoing metadata updates until
a full transaction commit is done. It's a performance
optimization of fsync. The log tracked in the tree are replayed
if the filesystem is not unmounted cleanly.

metadata
Data about data. In btrfs, this includes all of the internal
data structures of the filesystem, including directory
structures, filenames, file permissions, checksums, and the
location of each file's extents. All btrfs metadata is stored in
B-trees.

mkfs.btrfs
The tool (from btrfs-progs) to create a btrfs filesystem.

offline
A filesystem which is not mounted is offline. Some tools (e.g.
btrfsck) will only work on offline filesystems. Compare online.

online A filesystem which is mounted is online. Most btrfs tools will
only work on online filesystems. Compare offline.

orphan A file that's still in use (opened by a running process) but all
directory entries of that file have been removed.

RAID A class of different methods for writing some additional
redundant data across multiple devices so that if one device
fails, the missing data can be reconstructed from the remaining
ones. See RAID0, RAID1, RAID5, RAID6, RAID10, DUP and single.
Traditional RAID methods operate across multiple devices of
equal size, whereas btrfs' RAID implementation works inside
block groups.

RAID0 A form of RAID which provides no guarantees of error recovery,
but stripes a single copy of data across multiple devices for
performance purposes. The stripe size is fixed to 64KB for now.

RAID1, RAID1C3, RAID1C4
A form of RAID which stores two/three/four complete copies of
each piece of data. Each copy is stored on a different device.
btrfs requires a minimum of two devices to use RAID-1 or
three/four respectively. This is the default block group
profile for btrfs's metadata on more than one device.

RAID5 A form of RAID which stripes a single copy of data across
multiple devices, including one device's worth of additional
parity data. Can be used to recover from a single device
failure.

RAID6 A form of RAID which stripes a single copy of data across
multiple devices, including two device's worth of additional
parity data. Can be used to recover from the failure of two
devices.

RAID10 A form of RAID which stores two complete copies of each piece of
data, and also stripes each copy across multiple devices for
performance.

reflink
Commonly used as a reference to a shallow copy of file extents
that share the extents until the first change. Reflinked files
(e.g. by the cp) are different files but point to the same
extents, any change will be detected and new copy of the data
created, keeping the files independent. Related to that is
extent range cloning, that works on a range of a file.

relocation
The process of moving block groups within the filesystem while
maintaining full filesystem integrity and consistency. This
functionality is underlying balance and device removing
features.

scrub An online filesystem checking tool. Reads all the data and
metadata on the filesystem, verifies checksums and eventually
uses redundant copies from RAID or DUP repair any corrupt
data/metadata.

seed device
A readonly device can be used as a filesystem seed or template
(e.g. a CD-ROM containing an OS image). Read/write devices can
be added to store modifications (using copy on write), changes
to the writable devices are persistent across reboots. The
original device remains unchanged and can be removed at any time
(after Btrfs has been instructed to copy over all missing
blocks). Multiple read/write file systems can be built from the
same seed.

single A block group profile storing a single copy of each piece of
data.

snapshot
A subvolume which is a copy on write copy of another subvolume.
The two subvolumes share all of their common (unmodified) data,
which means that snapshots can be used to keep the historical
state of a filesystem very cheaply. After the snapshot is made,
the original subvolume and the snapshot are of equal status: the
original does not "own" the snapshot, and either one can be
deleted without affecting the other one.

subvolume
A tree of files and directories inside a btrfs that can be
mounted as if it were an independent filesystem. A subvolume is
created by taking a reference on the root of another subvolume.
Each btrfs filesystem has at least one subvolume, the top-level
subvolume, which contains everything else in the filesystem.
Additional subvolumes can be created and deleted with the btrfs<
tool. All subvolumes share the same pool of free space in the
filesystem. See also default subvolume.

super block
A special metadata block that is a main access point of the
filesystem structures. It's size is fixed and there are fixed
locations on the devices used for detecting and opening the
filesystem. Updating the superblock defines one transaction. The
super blocks contains filesystem identification (UUID), checksum
type, block pointers to fundamental trees, features and creation
parameters.

system array
A technical term for super block metadata describing how to
assemble a filesystem from multiple device, storing information
about chunks and devices that are required to be
scanned/registered at the time the mount happens. Scanning is
done by command btrfs device scan, alternatively all the
required devices can be specified by a mount option
device=/path.

top-level subvolume
The subvolume at the very top of the filesystem. This is the
only subvolume present in a newly-created btrfs filesystem, and
internally has ID 5, otherwise could be referenced as 0 (e.g.
within the set-default subcommand of btrfs).

transaction
A consistent set of changes. To avoid generating very large
amounts of disk activity, btrfs caches changes in RAM for up to
30 seconds (sometimes more often if the filesystem is running
short on space or doing a lot of fsync*s), and then writes
(commits) these changes out to disk in one go (using *copy on
write behaviour). This period of caching is called a
transaction. Only one transaction is active on the filesystem at
any one time.

transid
An alternative term for generation.

writeback
Writeback in the context of the Linux kernel can be defined as
the process of writing "dirty" memory from the page cache to the
disk, when certain conditions are met (timeout, number of dirty
pages over a ratio).

STORAGE MODEL, HARDWARE CONSIDERATIONS
Storage model
A storage model is a model that captures key physical aspects of data
structure in a data store. A filesystem is the logical structure
organizing data on top of the storage device.

The filesystem assumes several features or limitations of the storage
device and utilizes them or applies measures to guarantee reliability.
BTRFS in particular is based on a COW (copy on write) mode of writing,
i.e. not updating data in place but rather writing a new copy to a
different location and then atomically switching the pointers.

In an ideal world, the device does what it promises. The filesystem
assumes that this may not be true so additional mechanisms are applied
to either detect misbehaving hardware or get valid data by other means.
The devices may (and do) apply their own detection and repair
mechanisms but we won't assume any.

The following assumptions about storage devices are considered (sorted
by importance, numbers are for further reference):

1. atomicity of reads and writes of blocks/sectors (the smallest unit
of data the device presents to the upper layers)

2. there's a flush command that instructs the device to forcibly order
writes before and after the command; alternatively there's a barrier
command that facilitates the ordering but may not flush the data

3. data sent to write to a given device offset will be written without
further changes to the data and to the offset

4. writes can be reordered by the device, unless explicitly serialized
by the flush command

5. reads and writes can be freely reordered and interleaved

The consistency model of BTRFS builds on these assumptions. The logical
data updates are grouped, into a generation, written on the device,
serialized by the flush command and then the super block is written
ending the generation. All logical links among metadata comprising a
consistent view of the data may not cross the generation boundary.

When things go wrong
No or partial atomicity of block reads/writes (1)

o Problem: a partial block contents is written (torn write), e.g. due
to a power glitch or other electronics failure during the read/write

o Detection: checksum mismatch on read

o Repair: use another copy or rebuild from multiple blocks using some
encoding scheme

The flush command does not flush (2)

This is perhaps the most serious problem and impossible to mitigate by
filesystem without limitations and design restrictions. What could
happen in the worst case is that writes from one generation bleed to
another one, while still letting the filesystem consider the
generations isolated. Crash at any point would leave data on the device
in an inconsistent state without any hint what exactly got written,
what is missing and leading to stale metadata link information.

Devices usually honor the flush command, but for performance reasons
may do internal caching, where the flushed data are not yet
persistently stored. A power failure could lead to a similar scenario
as above, although it's less likely that later writes would be written
before the cached ones. This is beyond what a filesystem can take into
account. Devices or controllers are usually equipped with batteries or
capacitors to write the cache contents even after power is cut.
(Battery backed write cache)

Data get silently changed on write (3)

Such thing should not happen frequently, but still can happen
spuriously due the complex internal workings of devices or physical
effects of the storage media itself.

o Problem: while the data are written atomically, the contents get
changed

o Detection: checksum mismatch on read

o Repair: use another copy or rebuild from multiple blocks using some
encoding scheme

Data get silently written to another offset (3)

This would be another serious problem as the filesystem has no
information when it happens. For that reason the measures have to be
done ahead of time. This problem is also commonly called ghost write.

The metadata blocks have the checksum embedded in the blocks, so a
correct atomic write would not corrupt the checksum. It's likely that
after reading such block the data inside would not be consistent with
the rest. To rule that out there's embedded block number in the
metadata block. It's the logical block number because this is what the
logical structure expects and verifies.

The following is based on information publicly available, user
feedback, community discussions or bug report analyses. It's not
complete and further research is encouraged when in doubt.

Main memory
The data structures and raw data blocks are temporarily stored in
computer memory before they get written to the device. It is critical
that memory is reliable because even simple bit flips can have vast
consequences and lead to damaged structures, not only in the filesystem
but in the whole operating system.

Based on experience in the community, memory bit flips are more common
than one would think. When it happens, it's reported by the
tree-checker or by a checksum mismatch after reading blocks. There are
some very obvious instances of bit flips that happen, e.g. in an
ordered sequence of keys in metadata blocks. We can easily infer from
the other data what values get damaged and how. However, fixing that is
not straightforward and would require cross-referencing data from the
entire filesystem to see the scope.

If available, ECC memory should lower the chances of bit flips, but
this type of memory is not available in all cases. A memory test should
be performed in case there's a visible bit flip pattern, though this
may not detect a faulty memory module because the actual load of the
system could be the factor making the problems appear. In recent years
attacks on how the memory modules operate have been demonstrated
(rowhammer) achieving specific bits to be flipped. While these were
targeted, this shows that a series of reads or writes can affect
unrelated parts of memory.

Block group profiles with redundancy (like RAID1) will not protect
against memory errors as the blocks are first stored in memory before
they are written to the devices from the same source.

A filesystem mounted read-only will not affect the underlying block
device in almost 100% (with highly unlikely exceptions). The exception
is a tree-log that needs to be replayed during mount (and before the
read-only mount takes place), working memory is needed for that and
that can be affected by bit flips. There's a theoretical case where
bit flip changes the filesystem status from read-only to read-write.