btrfs-man5 - topics about the BTRFS filesystem (mount options,
   supported file attributes and other)


   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


   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

       (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.

           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

       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

       Force clearing and rebuilding of the disk space cache if something
       has gone wrong. See also: space_cache.

       (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.

           If compression is enabled, nodatacow and nodatasum are

   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

           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)).

           If nodatacow or nodatasum are enabled, compression is disabled.

       (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.

       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.

           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.

       (since: 3.4, default: bug)

       Action to take when encountering a fatal error.

           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() 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.

       (depends on compile-time option BTRFS_DEBUG, since: 4.4, default:

       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.

           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.

           currently, the tree log is replayed even with a read-only
           mount! To disable that behaviour, mount also with nologreplay.

       (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.

           the default value has changed to 2048 in kernel 4.6.

       (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).

       (since: 3.2, default: off, deprecated since: 4.5)

           this option has been replaced by usebackuproot and should not
           be used but will work on 4.5+ kernels.

       (since: 4.5, default: off)

       Do not attempt any data recovery at mount time. This will disable
       logreplay and avoids other write operations.

           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.

       (since: 3.12, default: off)

       Force check and rebuild procedure of the UUID tree. This should not
       normally be needed.

       (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=v2, nospace_cache
       (nospace_cache since: 3.2, space_cache=v2 since 4.5, default: on)

       Options to control the free space cache. This affects performance
       as searching for new free blocks could take longer if the space
       cache is not enabled. On the other hand, managing the space cache
       consumes some resources. It can be disabled without clearing at
       mount time.

       There are two implementations of how the space is tracked. The safe
       default is v1. On large filesystems (many-terabytes) and certain
       workloads the v1 performance may degrade. This problem is addressed
       by v2, that is based on b-trees, sometimes referred to as

       Compatibility notes:

       *   the v2 has to be enabled manually at mount time, once

       *   kernel without v2 support will be able to mount the filesystem
           in read-only mode

       *   v2 can be removed by mounting with clear_cache

   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

       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

       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.

           if both subvolid and subvol are specified, they must point at
           the same subvolume, otherwise mount will fail.

       (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.

       (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.

           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.

           This option has replaced recovery.

       (default: off)

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


   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

   There are several classes and the respective tools to manage the

   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 was had been enabled at the mount time and turned off

       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):

       (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.

       (since: 2.6.34)

       the default subvolume has been set on the filesystem

       (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

       (since: 2.6.31)

       the last major disk format change, improved backreferences

       (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

       (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

       (since: 3.9)

       the filesystem contains or contained a raid56 profile of block

       (since: 3.10)

       reduced-size metadata for extent references, saves a few percent of


   The btrfs filesystem supports setting the following file attributes
   using the chattr(1) utility:

       append only, new writes are always written at the end of the file

       no atime updates

       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

       no copy-on-write, file modifications are done in-place

       When set on a directory, all newly created files will inherit this

           due to implementation limitations, this flag can be set/unset
           only on empty files.

       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

       synchronous directory updates, for more details search open(2) for
       O_SYNC and O_DSYNC

       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)

       synchronous updates, for more details search open(2) for O_SYNC and

       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

   No other attributes are supported. For the complete list please refer
   to the chattr(1) manual page.


   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

       $ 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:

   *   scan devices for btrfs filesytem (ie. to let multi-device
       filesystems mount automatically) and register them with the kernel

   *   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

   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.


   acl(5), btrfs(8), chattr(1), fstrim(8), ioctl(2), mkfs.btrfs(8),


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