Ubuntu Manpage: btrfs-man5 - topics about the BTRFS filesystem (mount options, supported file attributes

Provided by: btrfs-tools_4.4-1ubuntu1.1_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. file attributes

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.

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 not practial 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 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 unwrittent 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.

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)).

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)).

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 severly 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.

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 hits 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: on)

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

Disabling flushing may improve performance but is not crash-safe.

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.

max_inline=bytes
(default: min(8192, 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, max inline data is ~3900 bytes.

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

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 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)

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.

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.

space_cache, nospace_cache
(nospace_cache since: 3.2, default: on)

Disable freespace cache loading without clearing the cache and the free space cache
will not be used during the mount. This affects performance as searching for new free
blocks could take longer. On the other hand, managing the space cache consumes some
resources.

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 nonrotational disk is
in use. 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.

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

user_subvol_rm_allowed
(default: off)

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

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.

NAME

DESCRIPTION

MOUNT OPTIONS

FILE ATTRIBUTES

SEE ALSO