Provided by: btrfs-progs_5.4.1-2_amd64 
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:
1. mount options
2. filesystem features
3. checksum algorithms
4. filesystem limits
5. bootloader support
6. file attributes
7. control device
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.
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 defaul 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 an 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, nodiscard
(default: off)
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. if the backing device does not support asynchronous queued
TRIM, then this operation 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.
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.
inode_cache, noinode_cache
(since: 3.0, default: off)
Enable free inode number caching. Not recommended to use unless files on your
filesystem get assigned inode numbers that are approaching 264. Normally, new files in
each subvolume get assigned incrementally (plus one from the last time) and are not
reused. The mount option turns on caching of the existing inode numbers and reuse of
inode numbers of deleted files.
This option may slow down your system at first run, or after mounting without the
option.
Note
Defaults to off due to a potential overflow problem when the free space checksums
don’t fit inside a single page.
Don’t use this option unless you really need it. The inode number limit on 64bit
system is 264, which is practically enough for the whole filesystem lifetime. Due to
implementation of linux VFS layer, the inode numbers on 32bit systems are only 32 bits
wide. This lowers the limit significantly and makes it possible to reach it. In such
case, this mount option will help. Alternatively, files with high inode numbers can be
copied to a new subvolume which will effectively start the inode numbers from the
beginning again.
logreplay, nologreplay
(default: on, even read-only)
Enable/disable log replay at mount time. 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.
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(8) command currently only has read-only support for v2. A read-write
command may be run on a v2 filesystem by clearing the cache, running the command, and
then remounting with space_cache=v2.
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, nousebackuproot
(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.
alloc_start=bytes
(default: 1M, minimum: 1M, deprecated since: 4.13)
Debugging option to force all block allocations above a certain byte threshold on each
block device. The value is specified in bytes, optionally with a K, M, or G suffix
(case insensitive).
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.
subvolrootid=objectid
(irrelevant since: 3.2, formally deprecated since: 3.10)
A workaround option from times (pre 3.2) when it was not possible to mount a subvolume
that did not reside directly under the toplevel subvolume.
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
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
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
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 subystem:
• filesystem - must be only single device
• 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.
# truncate -s 0 swapfile
# chattr +C swapfile
# fallocate -l 2G swapfile
# chmod 0600 swapfile
# mkswap swapfile
# swapon swapfile
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%" ]
┌────────┬─────────────┬───────┐
│ │ │ │
│Digest │ Cycles/4KiB │ Ratio │
├────────┼─────────────┼───────┤
│ │ │ │
│CRC32C │ 1700 │ 1.00 │
├────────┼─────────────┼───────┤
│ │ │ │
│XXHASH │ 2500 │ 1.44 │
├────────┼─────────────┼───────┤
│ │ │ │
│SHA256 │ 105000 │ 61 │
├────────┼─────────────┼───────┤
│ │ │ │
│BLAKE2b │ 22000 │ 13 │
└────────┴─────────────┴───────┘
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
264 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 264 (16 EiB) but the linux VFS limit is 263 (8 EiB)
maximum number of subvolumes
the subvolume ids can go up to 264 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
BOOTLOADER SUPPORT
GRUB2 (https://www.gnu.org/software/grub) has the most advanced support of booting from
BTRFS with respect to features.
EXTLINUX (from the https://syslinux.org project) can boot but does not support all
features. Please check the upstream documentation before you use it.
FILE ATTRIBUTES
The btrfs filesystem supports setting the following file attributes using the chattr(1)
utility:
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.
C
no copy-on-write, file 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)
S
synchronous updates, for more details search open(2) for O_SYNC and O_DSYNC
X
no compression, permanently turn off compression on the given file. Any compression
mount options will not affect this file.
When set on a directory, all newly created files will inherit this attribute.
No other attributes are supported. For the complete list please refer to the chattr(1)
manual page.
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 usually created by a system device node manager (eg. udev), but can be
created manually:
# mknod --mode=600 c 10 234 /dev/btrfs-control
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
acl(5), btrfs(8), chattr(1), fstrim(8), ioctl(2), mkfs.btrfs(8), mount(8), swapon(8)
2020-02-15 BTRFS-MAN5(5)