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BTRFS-MAN5(5) Btrfs Manual 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:
1. mount options
2. filesystem features
3. file attributes
4. 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).
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.
alloc_start=bytes
(default: 1M, minimum: 1M)
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).
This option was used for testing and has no practical use, it's slated to be removed
in the future.
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
ref-links 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 ref-links.
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 it's 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).
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 info.
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 commit. Higher values defer data being synced to
permanent storage with 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).
compress, compress=type, compress-force, compress-force=type
(default: off)
Control BTRFS file data compression. Type may be specified as zlib, lzo or no (for no
compression, used for remounting). If no type is specified, zlib is used. If
compress-force is specified, all files will be compressed, whether or not they
compress well. 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.
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.
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.
degraded
(default: off)
Allow mounts with less devices than the raid profile constraints require. A read-write
mount (or remount) may fail with too many devices missing, for example if a stripe
member is completely missing from RAID0.
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 using TRIM operation. This is useful for SSD
devices, thinly provisioned LUNs or virtual machine images where the backing device
understands the operation. Depending on support of the underlying device, the
operation may severely hurt performance in case the TRIM operation is synchronous (eg.
with SATA devices up to revision 3.0).
If discarding is not necessary to be done at the block freeing time, there's fstrim
tool that lets the filesystem discard all free blocks in a batch, possibly not much
interfering with other operations. Also, the the device may ignore the TRIM command if
the range is too small, so running the batch discard can actually discard 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.
logreplay, nologreplay
(default: on, even read-only)
Enable/disable log replay at mount time. See also treelog.
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, in bytes, 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).
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.
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
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 interrupted balance operation after mount. May be resumed
with btrfs balance resume or the paused state can be removed by btrfs balance cancel.
The default behaviour is to start interrutpd balance.
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 implementation,
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 as of 4.9.
ssd, nossd, ssd_spread
(default: SSD autodetected)
Options to control SSD allocation schemes. By default, BTRFS will enable or disable
SSD allocation heuristics depending on whether a rotational or non-rotational disk is
in use (contents of /sys/block/DEV/queue/rotational). The ssd and nossd options can
override this autodetection.
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.
subvol=path
Mount subvolume from path rather than the toplevel subvolume. The path is absolute
(ie. starts at 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 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 mount will fail.
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.
thread_pool=number
(default: min(NRCPUS + 2, 8) )
The number of worker threads to allocate. 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, 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, mount also 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
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.
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, 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 of is stored insid the
file. The value 1 is for enabled, 0 means the feature had been enabled at the mount
time and 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 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.
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
mixed_backref
(since: 2.6.31)
the last major disk format change, improved backreferences
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
skinny_metadata
(since: 3.10)
reduced-size metadata for extent references, saves a few percent of metadata
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 on 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, other compression
mount options will not affect that
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 filesyste
module:
o scan devices for btrfs filesystem (ie. to let multi-device filesystems mount
automatically) and register them with the kernel module
o similar to scan, but also wait until the device scanning process is finished for a
given filesystem
o get the supported features (can be also found under /sys/fs/btrfs/features)
The device is usually created by ..., but can be created manually:
# mknod --mode=600 c 10 234 /dev/btrfs-control
The 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)
Btrfs v4.9.1 08/06/2017 BTRFS-MAN5(5)
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