Provided by: btrfs-progs_4.16.1-2ubuntu1_amd64 bug

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. filesystem limits

        4. file attributes

        5. 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).

           Note
           most mount options apply to the whole filesystem and only options in the first mounted
           subvolume will take effect. This is due to lack of implementation and may change in
           the future. This means that (for example) you can’t set per-subvolume nodatacow,
           nodatasum, or compress using mount options. This should eventually be fixed, but it
           has proved to be difficult to implement correctly within the Linux VFS framework.

       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.

       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 reflinks
               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 reflinks.

       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 its 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). The main goal is to verify that all
           blocks from a given transaction period are properly linked.

           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 information.

       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 transaction commit when data are synchronized to
           permanent storage. Higher interval values lead to larger amount of unwritten data,
           which has 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). Use with care.

       compress, compress=type, compress-force, compress-force=type
           (default: off)

           Control BTRFS file data compression. Type may be specified as zlib, lzo, zstd or no
           (for no compression, used for remounting). If no type is specified, zlib is used. If
           compress-force is specified, then compression will always be attempted, but the data
           may end up uncompressed if the compression would make them larger.

           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. As this is too simple, the compress-force is a workaround
           that will compress most of the files at the cost of some wasted CPU cycles on failed
           attempts. The heuristics of compress will improve in the future so this will not be
           necessary.

               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.
           Updates in-place improve performance for workloads that do frequent overwrites, at the
           cost of potential partial writes, in case the write is interruted (system crash,
           device failure).

       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.
           There is a slight performance gain when checksums are turned off, the correspoinding
           metadata blocks holding the checksums do not need to updated. The cost of checksumming
           of the blocks in memory is much lower than the IO, modern CPUs feature hardware
           support of the checksumming algorithm.

       degraded
           (default: off)

           Allow mounts with less devices than the RAID profile constraints require. A read-write
           mount (or remount) may fail when there are too many devices missing, for example if a
           stripe member is completely missing from RAID0.

           Since 4.14, the constraint checks have been improved and are verified on the chunk
           level, not an the device level. This allows degraded mounts of filesystems with mixed
           RAID profiles for data and metadata, even if the device number constraints would not
           be satisfied for some of the prifles.

           Example: metadata — raid1, data — single, devices — /dev/sda, /dev/sdb

           Suppose the data are completely stored on sda, then missing sdb will not prevent the
           mount, even if 1 missing device would normally prevent (any) single profile to mount.
           In case some of the data chunks are stored on sdb, then the constraint of single/data
           is not satisfied and the filesystem cannot be mounted.

       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. This is useful for SSD devices, thinly
           provisioned LUNs, or virtual machine images; however, every storage layer must support
           discard for it to work. if the backing device does not support asynchronous queued
           TRIM, then this operation can severely degrade performance, because a synchronous TRIM
           operation will be attempted instead. Queued TRIM requires newer than SATA revision 3.1
           chipsets and devices.

       If it is not necessary to immediately discard freed blocks, then the fstrim tool can be
       used to discard all free blocks in a batch. Scheduling a TRIM during a period of low
       system activity will prevent latent interference with the performance of other operations.
       Also, a device may ignore the TRIM command if the range is too small, so running a batch
       discard has a greater probability of actually discarding the blocks.

       If discarding is not necessary to be done at the block freeing time, there’s fstrim(8)
       tool that lets the filesystem discard all free blocks in a batch, possibly not much
       interfering with other operations. Also, 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.
           Don’t use this option unless you really need it. The inode number limit on 64bit
           system is 264, which is practically enough for the whole filesystem lifetime. Due to
           implementation of linux VFS layer, the inode numbers on 32bit systems are only 32 bits
           wide. This lowers the limit significantly and makes it possible to reach it. In such
           case, this mount option will help. Alternatively, files with high inode numbers can be
           copied to a new subvolume which will effectively start the inode numbers from the
           beginning again.

       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, 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).

       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 an interrupted balance operation. The operation can later be
           resumed with btrfs balance resume, or the paused state can be removed with btrfs
           balance cancel. The default behaviour is to resume an interrupted balance immediately
           after a volume is mounted.

       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 one, referred to
           as 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.

       ssd, ssd_spread, nossd
           (default: SSD autodetected)

           Options to control SSD allocation schemes. By default, BTRFS will enable or disable
           SSD optimizations depending on status of a device with respect to rotational or
           non-rotational type. This is determined by the contents of
           /sys/block/DEV/queue/rotational). If it is 1, the ssd option is turned on. The option
           nossd will disable the autodetection.

           The optimizations make use of the absence of the seek penalty that’s inherent for the
           rotational devices. The blocks can be typically written faster and are not offloaded
           to separate threads.

               Note
               Since 4.14, the block layout optimizations have been dropped. This used to help
               with first generations of SSD devices. Their FTL (flash translation layer) was not
               effective and the optimization was supposed to improve the wear by better aligning
               blocks. This is no longer true with modern SSD devices and the optimization had no
               real benefit. Furthermore it caused increased fragmentation. The layout tuning has
               been kept intact for the option ssd_spread.
           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. The option nossd will disable all SSD
           options.

       subvol=path
           Mount subvolume from path rather than the toplevel subvolume. The path is always
           treated as relative to 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 of btrfs subvolume show 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 the mount will fail.

       thread_pool=number
           (default: min(NRCPUS + 2, 8) )

           The number of worker threads to start. 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, process scheduling, 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, also mount 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
           (since: 4.6, 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.

               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.

               Note
               historically, any user could create a snapshot even if he was not owner of the
               source subvolume, the subvolume deletion has been restricted for that reason. The
               subvolume creation has been restricted but this mount option is still required.
               This is a usability issue and will be addressed in the future.

   DEPRECATED MOUNT OPTIONS
       List of mount options that have been removed, kept for backward compatibility.

       alloc_start=bytes
           (default: 1M, minimum: 1M, deprecated since: 4.13)

           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).

       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.

       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.

   NOTES ON GENERIC MOUNT OPTIONS
       Some of the general mount options from mount(8) that affect BTRFS and are worth
       mentioning.

       noatime
           under read intensive work-loads, specifying noatime significantly improves performance
           because no new access time information needs to be written. Without this option, the
           default is relatime, which only reduces the number of inode atime updates in
           comparison to the traditional strictatime. The worst case for atime updates under
           relatime occurs when many files are read whose atime is older than 24 h and which are
           freshly snapshotted. In that case the atime is updated and COW happens - for each file
           - in bulk. See also https://lwn.net/Articles/499293/ - Atime and btrfs: a bad
           combination? (LWN, 2012-05-31).

           Note that noatime may break applications that rely on atime uptimes like the venerable
           Mutt (unless you use maildir mailboxes).

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 is stored inside the
           file. The value 1 is for enabled and active, while 0 means the feature was enabled at
           mount time but 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 for metadata blocks, this can be 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.

       compress_zstd
           (since: 4.14)

           the zstd 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, now default

       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

FILESYSTEM LIMITS

       maximum file name length
           255

       maximum symlink target length
           depends on the nodesize value, for 4k it’s 3949 bytes, for larger nodesize it’s 4095

           The symlink target may not be a valid path, ie the path name components can exceed the
           limits, there’s no content validation at symlink(3) creation.

       maximum number of inodes
           264 but depends on the available metadata space as the inodes are created dynamically

       inode numbers
           minimum number: 256 (for subvolumes), regular files and directories: 257

       maximum file length
           inherent limit of btrfs is 264 (16 EiB) but the linux VFS limit is 263 (8 EiB)

       maximum number of subvolumes
           264 but depends on the available metadata space, the space consumed by all subvolume
           metadata includes bookeeping of the shared extents can be large (MiB, GiB)

       maximum number of hardlinks of a file in a directory
           65536 when the extref feature is turned on during mkfs (default), roughly 100
           otherwise

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 but 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. Any compression
           mount options will not affect this file.

           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 filesystem
       module:

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

       ·   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 /sys/fs/btrfs/features)

       The device is usually created by a system device node manager (eg. udev), but can be
       created manually:

           # mknod --mode=600 c 10 234 /dev/btrfs-control

       The control 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)