btrfs-man5(5)
=============

NAME
----
btrfs-man5 - 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:

. mount options
. filesystem features
. checksum algorithms
. compression
. filesystem exclusive operations
. filesystem limits
. bootloader support
. file attributes
. zoned mode
. control device
. filesystems with multiple block group profiles
. seeding device
. raid56 status and recommended practices
. storage model
. hardware considerations


MOUNT OPTIONS
-------------

This section describes mount options specific to BTRFS.  For the generic mount
options please refer to `mount`(8) manpage. 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 eg.
'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 64K) are detected and queued up for the defragmentation process.
Not 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.

*check_int*::
*check_int_data*::
*check_int_print_mask='value'*::
(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 bitmask 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.

*clear_cache*::
Force clearing and rebuilding of the disk space cache if something
has gone wrong. 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, 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.

*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', ie. 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 less 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 (eg. run from the initial ramdisk). In
cases where this is not possible the 'device' mount option can help.
+
NOTE: booting eg. 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: off, async support since: 5.6)
+
Enable discarding of freed file blocks.  This is useful for SSD 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 newer than SATA revision 3.1 chipsets 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, eg. '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 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 '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 leaf headers.  For example, a 4k 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.

*rescue*::
(since: 5.9)
+
Modes allowing mount with damaged filesystem structures.
+
* 'usebackuproot' (since: 5.9, replaces standalone option 'usebackuproot')
* 'nologreplay' (since: 5.9, replaces standalone option 'nologreplay')
* 'ignorebadroots', 'ibadroots' (since: 5.11)
* 'ignoredatacsums', 'idatacsums' (since: 5.11)
* 'all' (since: 5.9)

*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 a volume 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=v1')
+
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', is the safe default. 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, kernels without 'v2'
support will only be able to mount the filesystem in read-only mode.
+
The `btrfs-check`(8) and `mkfs.btrfs`(8) 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 'v1'.

*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'.

*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 check
--clear-ino-cache*.


NOTES ON GENERIC MOUNT OPTIONS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some of the general mount options from `mount`(8) that affect BTRFS and are
worth mentioning.

*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 device* 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

*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, ie. 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

*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

*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 '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 'ZONED MODE'

SWAPFILE SUPPORT
~~~~~~~~~~~~~~~~

The swapfile is supported since kernel 5.0. Use `swapon`(8) to activate the
swapfile. There are some limitations of the implementation in btrfs and linux
swap subsystem:

* filesystem - must be only single device
* filesystem - must have only 'single' data profile
* swapfile - the containing subvolume cannot be snapshotted
* swapfile - must be preallocated
* swapfile - must be nodatacow (ie. also nodatasum)
* swapfile - must not be compressed

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 blocks once they've been attached to swap.

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

* balance - block groups with swapfile extents are skipped and reported, the rest will be processed normally
* resize grow - unaffected
* resize shrink - works as long as the extents are outside of the shrunk range
* device add - a new device does not interfere with existing swapfile and this operation will work, though no new swapfile can be activated afterwards
* device delete - if the device has been added as above, it can be also deleted
* device replace - ditto

When there are no active swapfiles and a whole-filesystem exclusive operation
is running (ie. 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
--------------------

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.

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
--------------------

CHECKSUM ALGORITHMS
-------------------

There are several checksum algorithms supported. The default and backward
compatible 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* (32bit digest)::
default, best backward compatibility, very fast, modern CPUs have
instruction-level support, not collision-resistant but still good error
detection capabilities

*XXHASH* (64bit digest)::
can be used as CRC32C successor, very fast, optimized for modern CPUs utilizing
instruction pipelining, good collision resistance and error detection

*SHA256* (256bit digest)::
a cryptographic-strength hash, relatively slow but with possible CPU
instruction acceleration or specialized hardware cards, FIPS certified and
in wide use

*BLAKE2b* (256bit digest)::
a 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
64bit platforms

The 'digest size' affects overall size of data block checksums stored in the
filesystem.  The metadata blocks have a fixed area up to 256bits (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 reference software implementations on a 3.5GHz intel CPU:

[ cols="^,>,>,>",width="50%" ]
|==================================================
h| Digest h| Cycles/4KiB h| Ratio h| Implementation
| CRC32C   |   1700       |   1.00 | CPU instruction
| XXHASH   |   2500       |   1.44 | reference impl.
| SHA256   | 105000       |     61 | reference impl.
| SHA256   |  36000       |     21 | libgcrypt/AVX2
| SHA256   |  63000       |     37 | libsodium/AVX2
| BLAKE2b  |  22000       |     13 | reference impl.
| BLAKE2b  |  19000       |     11 | libgcrypt/AVX2
| BLAKE2b  |  19000       |     11 | libsodium/AVX2
|==================================================

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). Basically, compression is on a file
by file basis. 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).

To enable compression, mount the filesystem with options 'compress' or
'compress-force'. Please refer to section 'MOUNT OPTIONS'. Once compression is
enabled, all new writes will be subject to compression. Some files may not
compress very well, and these are typically not recompressed but still written
uncompressed.

Each compression algorithm has different speed/ratio trade offs. The levels
can be selected by a mount option and affect only the resulting size (ie.
no compatibility issues).

Basic characteristics:

[horizontal]
ZLIB::
slower, higher compression ratio
* levels: 1 to 9, mapped directly, default level is 3
* good backward compatibility
LZO::
faster compression and decompression than zlib, worse compression ratio, designed to be fast
* no levels
* good backward compatibility
ZSTD::
compression comparable to zlib with higher compression/decompression speeds and different ratio
* levels: 1 to 15
* since 4.14, levels since 5.1

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.

The algorithms could be mixed in one file as they're stored per extent. The
compression can be changed on a file by *btrfs filesystem defrag* command,
using the '-c' option, or by *btrfs property set* using the 'compression'
property. Setting compression by 'chattr +c' utility will set it to zlib.

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:

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

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 WITH OTHER FEATURES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Compression is done using the COW mechanism so it's incompatible with
'nodatacow'. Direct IO works on compressed files but will fall back to buffered
writes. Currently 'nodatasum' and compression don't work together.


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.

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

- balance
- device add
- device delete
- device replace
- resize
- swapfile activate
- none

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


FILESYSTEM LIMITS
-----------------

maximum file name length::
255

maximum symlink target length::
depends on the 'nodesize' value, for 4k 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, ie. the path name components
can exceed the limits (NAME_MAX), there's no content validation at `symlink`(3)
creation.

maximum number of inodes::
2^64^ but depends on the available metadata space as the inodes are created
dynamically

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

maximum file length::
inherent limit of btrfs is 2^64^ (16 EiB) but the linux VFS limit is 2^63^ (8 EiB)

maximum number of subvolumes::
the subvolume ids can go up to 2^64^ 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)

maximum number of hardlinks of a file in a directory::
65536 when the `extref` feature is turned on during mkfs (default), roughly
100 otherwise

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.

EXTLINUX (from the https://syslinux.org project) can boot but does not support
all features. Please check the upstream documentation before you use it.

The first 1MiB on each device is unused with the exception of primary
superblock that is on the offset 64KiB and spans 4KiB.


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:

* '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
* '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` 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 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
~~~~~~~~~~~~~~~~~~~~~~~~~

* all devices must have the same zone size
* maximum zone size is 8GiB
* mixing zoned and non-zoned devices is possible, the zone writes are emulated,
  but this is namely for testing
* the super block is handled in a special way and is at different locations
  than on a non-zoned filesystem:
  * primary: 0B (and the next two zones)
  * secondary: 512G (and the next two zones)
  * 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 incompatbile with some features:

* nodatacow - overwrite in-place, cannot create such files
* fallocate - preallocating space for in-place first write
* mixed-bg - unordered writes to data and metadata, fixing that means using
  separate data and metadata block groups
* 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:

* only single profile is supported
* 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 0, 512G 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 eg. 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.


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:

* scan devices for btrfs filesystem (ie. to let multi-device filesystems mount
  automatically) and register them with the kernel module
* similar to scan, but also wait until the device scanning process is finished
  for a given filesystem
* 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
eg. 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
RAID6, RAID5, 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 single
device filesystem 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, 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, something like 'remount -o remount,rw /path' makes the filesystem at
'/path' ready for use. The simplest usecase is to throw away all changes by
unmounting the filesystem when convenient.

Alternatively, 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*, eg.
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 usecases, not for others, and a 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
# mount -o remount,rw /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
# mount -o remount,rw /mnt/mnt2
# ... /mnt/mnt2 is now writable
---------------

The writable device ('/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:

* 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
* block group profiles 'single' and 'dup' support the usecases above
* the label is copied from the seeding device and can be changed by *btrfs filesystem label*
* each new mount of the seeding device gets a new random UUID


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 (eg. 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, eg. 'df', 'btrfs filesystem df' or 'btrfs filesystem
usge'. When there's only a one profile per block group type (eg. 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.


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, ie. 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)**

- 'Problem': a partial block contents is written ('torn write'), eg. due to a
  power glitch or other electronics failure during the read/write
- 'Detection': checksum mismatch on read
- '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.

* 'Problem': while the data are written atomically, the contents get changed
* 'Detection': checksum mismatch on read
* '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.


HARDWARE CONSIDERATIONS
-----------------------

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.

Further reading:

- https://en.wikipedia.org/wiki/Row_hammer

What to do:

- run 'memtest', note that sometimes memory errors happen only when the system
  is under heavy load that the default memtest cannot trigger
- memory errors may appear as filesystem going read-only due to "pre write"
  check, that verify meta data before they get written but fail some basic
  consistency checks

DIRECT MEMORY ACCESS (DMA)
~~~~~~~~~~~~~~~~~~~~~~~~~~

Another class of errors is related to DMA (direct memory access) performed
by device drivers. While this could be considered a software error, the
data transfers that happen without CPU assistance may accidentally corrupt
other pages. Storage devices utilize DMA for performance reasons, the
filesystem structures and data pages are passed back and forth, making
errors possible in case page life time is not properly tracked.

There are lots of quirks (device-specific workarounds) in Linux kernel
drivers (regarding not only DMA) that are added when found. The quirks
may avoid specific errors or disable some features to avoid worse problems.

What to do:

- use up-to-date kernel (recent releases or maintained long term support versions)
- as this may be caused by faulty drivers, keep the systems up-to-date

ROTATIONAL DISKS (HDD)
~~~~~~~~~~~~~~~~~~~~~~

Rotational HDDs typically fail at the level of individual sectors or small clusters.
Read failures are caught on the levels below the filesystem and are returned to
the user as 'EIO - Input/output error'. Reading the blocks repeatedly may
return the data eventually, but this is better done by specialized tools and
filesystem takes the result of the lower layers. Rewriting the sectors may
trigger internal remapping but this inevitably leads to data loss.

Disk firmware is technically software but from the filesystem perspective is
part of the hardware. IO requests are processed, and caching or various
other optimizations are performed, which may lead to bugs under high load or
unexpected physical conditions or unsupported use cases.

Disks are connected by cables with two ends, both of which can cause problems
when not attached properly. Data transfers are protected by checksums and the
lower layers try hard to transfer the data correctly or not at all. The errors
from badly-connecting cables may manifest as large amount of failed read or
write requests, or as short error bursts depending on physical conditions.

What to do:

- check 'smartctl' for potential issues

SOLID STATE DRIVES (SSD)
~~~~~~~~~~~~~~~~~~~~~~~~

The mechanism of information storage is different from HDDs and this affects
the failure mode as well. The data are stored in cells grouped in large blocks
with limited number of resets and other write constraints. The firmware tries
to avoid unnecessary resets and performs optimizations to maximize the storage
media lifetime. The known techniques are deduplication (blocks with same
fingerprint/hash are mapped to same physical block), compression or internal
remapping and garbage collection of used memory cells. Due to the additional
processing there are measures to verity the data e.g. by ECC codes.

The observations of failing SSDs show that the whole electronic fails at once
or affects a lot of data (eg. stored on one chip). Recovering such data
may need specialized equipment and reading data repeatedly does not help as
it's possible with HDDs.

There are several technologies of the memory cells with different
characteristics and price. The lifetime is directly affected by the type and
frequency of data written.  Writing "too much" distinct data (e.g. encrypted)
may render the internal deduplication ineffective and lead to a lot of rewrites
and increased wear of the memory cells.

There are several technologies and manufacturers so it's hard to describe them
but there are some that exhibit similar behaviour:

- expensive SSD will use more durable memory cells and is optimized
for reliability and high load
- cheap SSD is projected for a lower load ("desktop user") and is optimized for
cost, it may employ the optimizations and/or extended error reporting partially
or not at all

It's not possible to reliably determine the expected lifetime of an SSD due to
lack of information about how it works or due to lack of reliable stats provided
by the device.

Metadata writes tend to be the biggest component of lifetime writes to a SSD,
so there is some value in reducing them. Depending on the device class (high
end/low end) the features like DUP block group profiles may affect the
reliability in both ways:

- 'high end' are typically more reliable and using 'single' for data and metadata
could be suitable to reduce device wear
- 'low end' could lack ability to identify errors so an additional
redundancy at the filesystem level (checksums, 'DUP') could help

Only users who consume 50 to 100% of the SSD's actual lifetime writes need to be
concerned by the write amplification of btrfs DUP metadata. Most users will be
far below 50% of the actual lifetime, or will write the drive to death and
discover how many writes 100% of the actual lifetime was. SSD firmware often
adds its own write multipliers that can be arbitrary and unpredictable and
dependent on application behavior, and these will typically have far greater
effect on SSD lifespan than DUP metadata. It's more or less impossible to
predict when a SSD will run out of lifetime writes to within a factor of two, so
it's hard to justify wear reduction as a benefit.

Further reading:

- https://www.snia.org/educational-library/ssd-and-deduplication-end-spinning-disk-2012
- https://www.snia.org/educational-library/realities-solid-state-storage-2013-2013
- https://www.snia.org/educational-library/ssd-performance-primer-2013
- https://www.snia.org/educational-library/how-controllers-maximize-ssd-life-2013

What to do:

- run 'smartctl' or self-tests to look for potential issues
- keep the firmware up-to-date

NVM EXPRESS, NON-VOLATILE MEMORY (NVMe)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NVMe is a type of persistent memory usually connected over a system bus (PCIe)
or similar interface and the speeds are an order of magnitude faster than SSD.
It is also a non-rotating type of storage, and is not typically connected by a
cable. It's not a SCSI type device either but rather a complete specification
for logical device interface.

In a way the errors could be compared to a combination of SSD class and regular
memory. Errors may exhibit as random bit flips or IO failures. There are tools
to access the internal log ('nvme log' and 'nvme-cli') for a more detailed
analysis.

There are separate error detection and correction steps performed e.g. on the
bus level and in most cases never making in to the filesystem level. Once this
happens it could mean there's some systematic error like overheating or bad
physical connection of the device. You may want to run self-tests (using
'smartctl').

* https://en.wikipedia.org/wiki/NVM_Express
* https://www.smartmontools.org/wiki/NVMe_Support

DRIVE FIRMWARE
~~~~~~~~~~~~~~

Firmware is technically still software but embedded into the hardware. As all
software has bugs, so does firmware. Storage devices can update the firmware
and fix known bugs. In some cases the it's possible to avoid certain bugs by
quirks (device-specific workarounds) in Linux kernel.

A faulty firmware can cause wide range of corruptions from small and localized
to large affecting lots of data. Self-repair capabilities may not be sufficient.

What to do:

- check for firmware updates in case there are known problems, note that
  updating firmware can be risky on itself
- use up-to-date kernel (recent releases or maintained long term support versions)

SD FLASH CARDS
~~~~~~~~~~~~~~

There are a lot of devices with low power consumption and thus using storage
media based on low power consumption too, typically flash memory stored on
a chip enclosed in a detachable card package. An improperly inserted card may be
damaged by electrical spikes when the device is turned on or off. The chips
storing data in turn may be damaged permanently. All types of flash memory
have a limited number of rewrites, so the data are internally translated by FTL
(flash translation layer). This is implemented in firmware (technically a
software) and prone to bugs that manifest as hardware errors.

Adding redundancy like using DUP profiles for both data and metadata can help
in some cases but a full backup might be the best option once problems appear
and replacing the card could be required as well.

HARDWARE AS THE MAIN SOURCE OF FILESYSTEM CORRUPTIONS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**If you use unreliable hardware and don't know about that, don't blame the
filesystem when it tells you.**


SEE ALSO
--------
`acl`(5),
`btrfs`(8),
`chattr`(1),
`fstrim`(8),
`ioctl`(2),
`mkfs.btrfs`(8),
`mount`(8),
`swapon`(8)
