btrfs

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. checksum algorithms

        4. filesystem limits

        5. bootloader support

        6. file attributes

        7. control device

        8. filesystems with multiple block group profiles

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.

       Mount options are processed in order, only the last occurence of an
       option takes effect and may disable other options due to constraints
       (see eg. nodatacow and compress). The output of mount command shows
       which options have been applied.

       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[:level], compress-force,
       compress-force=type[:level]
           (default: off, level support since: 5.1)

           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.

           Both zlib and zstd (since version 5.1) expose the compression level
           as a tunable knob with higher levels trading speed and memory
           (zstd) for higher compression ratios. This can be set by appending
           a colon and the desired level. Zlib accepts the range [1, 9] and
           zstd accepts [1, 15]. If no level is set, both currently use a
           default level of 3. The value 0 is an alias for the default level.

           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. Since kernel 4.15, a set of
           heuristic algorithms have been improved by using frequency
           sampling, repeated pattern detection and Shannon entropy
           calculation to avoid that.

               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 interrupted (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 corresponding 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 profiles.

           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, discard=sync, discard=async, nodiscard
           (default: off, async support since: 5.6)

           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.

           In the synchronous mode (sync or without option value), lack of
           asynchronous queued TRIM on the backing device TRIM 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.

           The asynchronous mode (async) gathers extents in larger chunks
           before sending them to the devices for TRIM. The overhead and
           performance impact should be negligible compared to the previous
           mode and it’s supposed to be the preferred mode if needed.

           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.

       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 2^64. 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 2^64, 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.

       nologreplay
           (default: off, even read-only)

           The tree-log contains pending updates to the filesystem until the
           full commit. The log is replayed on next mount, this can be
           disabled by this option. See also treelog. Note that nologreplay is
           the same as norecovery.

               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 that this option
           is the same as nologreplay.

               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, nossd_spread
           (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 0, 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 while
           nossd_spread only disables ssd_spread.

       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. Since 4.18, the rmdir(2) syscall can
               delete an empty subvolume just like an ordinary directory.
               Whether this is possible can be detected at runtime, see
               rmdir_subvol feature in FILESYSTEM FEATURES.

   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 or
           checksum algorithm, 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

       free_space_tree
           (since: 4.5)

           free space representation using a dedicated b-tree, successor of v1
           space cache

       metadata_uuid
           (since: 5.0)

           the main filesystem UUID is the metadata_uuid, which stores the new
           UUID only in the superblock while all metadata blocks still have
           the UUID set at mkfs time, see btrfstune(8) for more

       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

       raid1c34
           (since: 5.5)

           extended RAID1 mode with copies on 3 or 4 devices respectively

       raid56
           (since: 3.9)

           the filesystem contains or contained a raid56 profile of block
           groups

       rmdir_subvol
           (since: 4.18)

           indicate that rmdir(2) syscall can delete an empty subvolume just
           like an ordinary directory. Note that this feature only depends on
           the kernel version.

       skinny_metadata
           (since: 3.10)

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

       supported_checksums
           (since: 5.5)

           list of checksum algorithms supported by the kernel module, the
           respective modules or built-in implementing the algorithms need to
           be present to mount the filesystem

   SWAPFILE SUPPORT
       The swapfile is supported since kernel 5.0. Use swapon(8) to activate
       the swapfile. There are some limitations of the implementation in btrfs
       and linux swap subsystem:

       •   filesystem - must be only single device

       •   swapfile - the containing subvolume cannot be snapshotted

       •   swapfile - must be preallocated

       •   swapfile - must be nodatacow (ie. also nodatasum)

       •   swapfile - must not be compressed

       The limitations come namely from the COW-based design and mapping layer
       of blocks that allows the advanced features like relocation and
       multi-device filesystems. However, the swap subsystem expects simpler
       mapping and no background changes of the file blocks once they’ve been
       attached to swap.

       With active swapfiles, the following whole-filesystem operations will
       skip swapfile extents or may fail:

       •   balance - block groups with swapfile extents are skipped and
           reported, the rest will be processed normally

       •   resize grow - unaffected

       •   resize shrink - works as long as the extents are outside of the
           shrunk range

       •   device add - a new device does not interfere with existing swapfile
           and this operation will work, though no new swapfile can be
           activated afterwards

       •   device delete - if the device has been added as above, it can be
           also deleted

       •   device replace - ditto

       When there are no active swapfiles and a whole-filesystem exclusive
       operation is running (ie. balance, device delete, shrink), the
       swapfiles cannot be temporarily activated. The operation must finish
       first.

           # truncate -s 0 swapfile
           # chattr +C swapfile
           # fallocate -l 2G swapfile
           # chmod 0600 swapfile
           # mkswap swapfile
           # swapon swapfile

CHECKSUM ALGORITHMS
       There are several checksum algorithms supported. The default and
       backward compatible is crc32c. Since kernel 5.5 there are three more
       with different characteristics and trade-offs regarding speed and
       strength. The following list may help you to decide which one to
       select.

       CRC32C (32bit digest)
           default, best backward compatibility, very fast, modern CPUs have
           instruction-level support, not collision-resistant but still good
           error detection capabilities

       XXHASH (64bit digest)
           can be used as CRC32C successor, very fast, optimized for modern
           CPUs utilizing instruction pipelining, good collision resistance
           and error detection

       SHA256 (256bit digest)
           a cryptographic-strength hash, relatively slow but with possible
           CPU instruction acceleration or specialized hardware cards, FIPS
           certified and in wide use

       BLAKE2b (256bit digest)
           a cryptographic-strength hash, relatively fast with possible CPU
           acceleration using SIMD extensions, not standardized but based on
           BLAKE which was a SHA3 finalist, in wide use, the algorithm used is
           BLAKE2b-256 that’s optimized for 64bit platforms

       The digest size affects overall size of data block checksums stored in
       the filesystem. The metadata blocks have a fixed area up to 256bits (32
       bytes), so there’s no increase. Each data block has a separate checksum
       stored, with additional overhead of the b-tree leaves.

       Approximate relative performance of the algorithms, measured against
       CRC32C using reference software implementations on a 3.5GHz intel CPU:

       ┌────────┬─────────────┬───────┐
       │        │             │       │
       │Digest  Cycles/4KiB Ratio │
       ├────────┼─────────────┼───────┤
       │        │             │       │
       │CRC32C  │        1700 │  1.00 │
       ├────────┼─────────────┼───────┤
       │        │             │       │
       │XXHASH  │        2500 │  1.44 │
       ├────────┼─────────────┼───────┤
       │        │             │       │
       │SHA256  │      105000 │    61 │
       ├────────┼─────────────┼───────┤
       │        │             │       │
       │BLAKE2b │       22000 │    13 │
       └────────┴─────────────┴───────┘

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 due to the system limit PATH_MAX

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

       maximum number of inodes
           2^64 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 2^64 (16 EiB) but the linux VFS limit is
           2^63 (8 EiB)

       maximum number of subvolumes
           the subvolume ids can go up to 2^64 but the number of actual
           subvolumes depends on the available metadata space, the space
           consumed by all subvolume metadata includes bookkeeping of 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

BOOTLOADER SUPPORT
       GRUB2 (https://www.gnu.org/software/grub) has the most advanced support
       of booting from BTRFS with respect to features.

       U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for
       booting but not all BTRFS features are implemented, check the
       documentation.

       EXTLINUX (from the https://syslinux.org project) can boot but does not
       support all features. Please check the upstream documentation before
       you use it.

       The first 1MiB on each device is unused with the exception of primary
       superblock that is on the offset 64KiB and spans 4KiB.

FILE ATTRIBUTES
       The btrfs filesystem supports setting file attributes or flags. Note
       there are old and new interfaces, with confusing names. The following
       list should clarify that:

       •   attributes: chattr(1) or lsattr(1) utilities (the ioctls are
           FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the
           attributes are also called flags

       •   xflags: to distinguish from the previous, it’s extended flags, with
           tunable bits similar to the attributes but extensible and new bits
           will be added in the future (the ioctls are FS_IOC_FSGETXATTR and
           FS_IOC_FSSETXATTR but they are not related to extended attributes
           that are also called xattrs), there’s no standard tool to change
           the bits, there’s support in xfs_io(8) as command xfs_io -c chattr

   ATTRIBUTES
       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 data 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.

   XFLAGS
       There’s overlap of letters assigned to the bits with the attributes,
       this list refers to what xfs_io(8) provides:

       i
           immutable, same as the attribute

       a
           append only, same as the attribute

       s
           synchronous updates, same as the atribute S

       A
           no atime updates, same as the attribute

       d
           no dump, same as the attribute

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.

FILESYSTEM WITH MULTIPLE PROFILES
       It is possible that a btrfs filesystem contains multiple block group
       profiles of the same type. This could happen when a profile conversion
       using balance filters is interrupted (see btrfs-balance(8)). Some btrfs
       commands perform a test to detect this kind of condition and print a
       warning like this:

           WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
           WARNING:   Data: single, raid1
           WARNING:   Metadata: single, raid1

       The corresponding output of btrfs filesystem df might look like:

           WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
           WARNING:   Data: single, raid1
           WARNING:   Metadata: single, raid1
           Data, RAID1: total=832.00MiB, used=0.00B
           Data, single: total=1.63GiB, used=0.00B
           System, single: total=4.00MiB, used=16.00KiB
           Metadata, single: total=8.00MiB, used=112.00KiB
           Metadata, RAID1: total=64.00MiB, used=32.00KiB
           GlobalReserve, single: total=16.25MiB, used=0.00B

       There’s more than one line for type Data and Metadata, while the
       profiles are single and RAID1.

       This state of the filesystem OK but most likely needs the
       user/administrator to take an action and finish the interrupted tasks.
       This cannot be easily done automatically, also the user knows the
       expected final profiles.

       In the example above, the filesystem started as a single device and
       single block group profile. Then another device was added, followed by
       balance with convert=raid1 but for some reason hasn’t finished.
       Restarting the balance with convert=raid1 will continue and end up with
       filesystem with all block group profiles RAID1.

           Note
           If you’re familiar with balance filters, you can use
           convert=raid1,profiles=single,soft, which will take only the
           unconverted single profiles and convert them to raid1. This may
           speed up the conversion as it would not try to rewrite the already
           convert raid1 profiles.

       Having just one profile is desired as this also clearly defines the
       profile of newly allocated block groups, otherwise this depends on
       internal allocation policy. When there are multiple profiles present,
       the order of selection is RAID6, RAID5, RAID10, RAID1, RAID0 as long as
       the device number constraints are satisfied.

       Commands that print the warning were chosen so they’re brought to user
       attention when the filesystem state is being changed in that regard.
       This is: device add, device delete, balance cancel, balance pause.
       Commands that report space usage: filesystem df, device usage. The
       command filesystem usage provides a line in the overall summary:

               Multiple profiles:                 yes (data, metadata)

SEE ALSO
       acl(5), btrfs(8), chattr(1), fstrim(8), ioctl(2), mkfs.btrfs(8),
       mount(8), swapon(8)



Btrfs v5.7                        07/03/2020                     BTRFS-MAN5(5)