Provided by: btrfs-tools_4.4-1ubuntu1.1_amd64 bug


       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. file attributes


       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

       acl, noacl
           (default: on)

           Enable/disable support for Posix Access Control Lists (ACLs). See the acl(5) manual
           page for more information about ACLs.

           (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.

               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

           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

           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.

           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.

               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)).

           (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

       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.

           (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: 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

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

           (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

       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

               Defaults to off due to a potential overflow problem when the free space checksums
               don’t fit inside a single page.

           (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.

           (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).

           (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.

           (since: 3.12, default: off)

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

           (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

       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.

           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.

           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.

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

           (default: off)

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


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

           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

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

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

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

               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 O_DSYNC

           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.


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