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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