Provided by: btrfs-progs_6.2-1_amd64 bug


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


       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.  compression

       5.  sysfs interface

       6.  filesystem exclusive operations

       7.  filesystem limits

       8.  bootloader support

       9.  file attributes

       10. zoned mode

       11. control device

       12. filesystems with multiple block group profiles

       13. seeding device

       14. RAID56 status and recommended practices

       15. storage model, hardware considerations


       This  section  describes  mount  options specific to BTRFS.  For the generic mount options
       please refer to mount(8) manual page. The options are  sorted  alphabetically  (discarding
       the no prefix).

          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 occurrence of an option takes effect
       and may disable other options due to constraints (see e.g.  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  64KiB)  are  detected  and
              queued  up  for  the  defragmentation  process.  Not well suited for large database

              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

                 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

              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.

              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  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.

                 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)).

                 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,  i.e.
              the  normal  mode  of  operation. All files created under nodatasum inherit the "no
              checksums"  property,  however  there's  no  corresponding  file   attribute   (see

                 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.

              (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  at  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.

              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 (e.g. run from the initial ramdisk). In cases where  this
              is not possible the device mount option can help.

                 Booting  e.g.  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, 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.

              (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, e.g. panic, oops or crashkernel.

       flushoncommit, noflushoncommit
              (default: off)

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

              This makes the committed state a fully consistent view of the file system from  the
              application's  perspective (i.e. it includes all completed file system operations).
              This was previously the behavior only when a snapshot was created.

              When off, the filesystem is consistent but buffered writes may last more  than  one
              transaction commit.

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

              (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.

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

              (default: min(2048, page size) )

              Specify the maximum amount of space, 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  4KiB
              sectorsize, maximum size of inline data is about 3900 bytes.

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

                 The default value has changed to 2048 in kernel 4.6.

              (default: 0, internal logic)

              Specifies that 1 metadata chunk should be allocated after every value data  chunks.
              Default  behaviour depends on internal logic, some percent of unused metadata space
              is attempted to be maintained but is not always  possible  if  there's  not  enough
              space  left  for  chunk  allocation.  The  option  could  be useful to override the
              internal logic in favor of the metadata allocation  if  the  expected  workload  is
              supposed to be metadata intense (snapshots, reflinks, xattrs, inlined files).

              (since: 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.

                 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

              (since: 3.12, default: off)

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

       rescue (since: 5.9)

              Modes allowing mount with damaged filesystem structures.

              • usebackuproot (since: 5.9, replaces standalone option usebackuproot)

              • nologreplay (since: 5.9, replaces standalone option nologreplay)

              • ignorebadroots, ibadroots (since: 5.11)

              • ignoredatacsums, idatacsums (since: 5.11)

              • all (since: 5.9)

              (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:

              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

              The btrfs-check(8) and :doc:`mkfs.btrfs(8)<mkfs.btrfs> commands have full  v2  free
              space cache support since v4.19.

              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.

                 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.

              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.

              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

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

              (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.

                 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.

              (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.

                 This option has replaced recovery.

              (default: off)

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

                 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

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

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

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

       inode_cache, noinode_cache
              (removed in: 5.11, since: 3.0, default: off)

                 The  functionality  has been removed in 5.11, any stale data created by previous
                 use of the inode_cache option can be removed by btrfs check --clear-ino-cache.

       Some of the  general  mount  options  from  mount(8)  that  affect  BTRFS  and  are  worth

              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 -
              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).


       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
              /dev/sdx 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):

              (since: 3.4)

              the filesystem uses nodesize for metadata blocks, this can be bigger than the  page

              (since: 2.6.38)

              the  lzo  compression  has been used on the filesystem, either as a mount option or
              via btrfs filesystem defrag.

              (since: 4.14)

              the zstd compression has been used on the filesystem, either as a mount  option  or
              via btrfs filesystem defrag.

              (since: 2.6.34)

              the default subvolume has been set on the filesystem

              (since: 3.7)

              increased  hardlink limit per file in a directory to 65536, older kernels supported
              a varying number of hardlinks depending on the sum of all file name sizes that  can
              be stored into one metadata block

              (since: 4.5)

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

              (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

              (since: 2.6.31)

              the last major disk format change, improved backreferences, now default

              (since: 2.6.37)

              mixed  data and metadata block groups, i.e. the data and metadata are not separated
              and occupy the same block groups, this mode is suitable for small volumes as  there
              are  no  constraints  how the remaining space should be used (compared to the split
              mode, where empty metadata space cannot be used for data and vice versa)

              on the other hand, the final layout is  quite  unpredictable  and  possibly  highly
              fragmented, which means worse performance

              (since: 3.14)

              improved representation of file extents where holes are not explicitly stored as an
              extent, saves a few percent of metadata if sparse files are used

              (since: 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

              (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.

              (since: 3.10)

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

              (since: 5.10)

              number of the highest supported send stream version

              (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,
              see CHECKSUM ALGORITHMS

              (since: 5.13)

              list  of  values that are accepted as sector sizes (mkfs.btrfs --sectorsize) by the
              running kernel

              (since: 5.11)

              list of values for the mount option  rescue  that  are  supported  by  the  running
              kernel, see btrfs(5)

       zoned  (since: 5.12)

              zoned  mode  is allocation/write friendly to host-managed zoned devices, allocation
              space is partitioned into fixed-size zones that must be updated  sequentially,  see
              ZONED MODE


       A  swapfile is file-backed memory that the system uses to temporarily offload the RAM.  It
       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

       • filesystem - must have only single data profile

       • swapfile - the containing subvolume cannot be snapshotted

       • swapfile - must be preallocated (i.e. no holes)

       • swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)

       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 block
       location once they've been assigned 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
       (e.g. balance, device delete, shrink), the swapfiles cannot be temporarily activated.  The
       operation must finish first.

       To create and activate a swapfile run the following commands:

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

       Since  version  6.1  it's  possible to create the swapfile in a single command (except the

          # btrfs filesystem mkswapfile swapfile
          # swapon swapfile

       Please note that the UUID returned by the mkswap utility identifies the swap  "filesystem"
       and  because it's stored in a file, it's not generally visible and usable as an identifier
       unlike if it was on a block device.

       The file will appear in /proc/swaps:

          # cat /proc/swaps
          Filename          Type          Size           Used      Priority
          /path/swapfile    file          2097152        0         -2

       The swapfile can be created as one-time operation or, once properly created, activated  on
       each  boot  by  the  swapon  -a  command (usually started by the service manager). Add the
       following entry to /etc/fstab, assuming the filesystem that provides the  /path  has  been
       already  mounted at this point.  Additional mount options relevant for the swapfile can be
       set too (like priority, not the BTRFS mount options).

          /path/swapfile        none        swap        defaults      0 0


       A swapfile can be used for hibernation but it's not straightforward. Before hibernation  a
       resume  offset must be written to file /sys/power/resume_offset or the kernel command line
       parameter resume_offset must be set.

       The value is the physical offset on the device. Note that this is not the same value  that
       filefrag prints as physical offset!

       Btrfs filesystem uses mapping between logical and physical addresses but here the physical
       can still  map  to  one  or  more  device-specific  physical  block  addresses.  It's  the
       device-specific physical offset that is suitable as resume offset.

       Since  version  6.1  there's a command btrfs inspect-internal map-swapfile that will print
       the device physical offset and the adjusted value for /sys/power/resume_offset.  Note that
       the value is divided by page size, i.e. it's not the offset itself.

          # btrfs filesystem mkswapfile swapfile
          # btrfs inspect-internal map-swapfile swapfile
          Physical start: 811511726080
          Resume offset:     198122980

       For scripting and convenience the option -r will print just the offset:

          # btrfs inspect-internal map-swapfile -r swapfile

       The command map-swapfile also verifies all the requirements, i.e. no holes, single device,


       If the swapfile activation fails please verify that you followed all the  steps  above  or
       check the system log (e.g. dmesg or journalctl) for more information.

       Notably, the swapon utility exits with a message that does not say what failed:

          # swapon /path/swapfile
          swapon: /path/swapfile: swapon failed: Invalid argument

       The specific reason is likely to be printed to the system log by the btrfs module:

          # journalctl -t kernel | grep swapfile
          kernel: BTRFS warning (device sda): swapfile must have single data profile


       Data  and metadata are checksummed by default, the checksum is calculated before write and
       verified after reading the blocks from devices. The whole metadata block  has  a  checksum
       stored inline in the b-tree node header, each data block has a detached checksum stored in
       the checksum tree.

       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 256 bits (32 bytes), so there's no increase.
       Each data block has a separate checksum stored, with additional  overhead  of  the  b-tree

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

                           │Digest  │ Cycles/4KiB │ Ratio │ Implementation  │
                           │CRC32C  │ 1700        │ 1.00  │ CPU instruction │
                           │XXHASH  │ 2500        │ 1.44  │ reference impl. │
                           │SHA256  │ 105000      │ 61    │ reference impl. │
                           │SHA256  │ 36000       │ 21    │ libgcrypt/AVX2  │
                           │SHA256  │ 63000       │ 37    │ libsodium/AVX2  │
                           │BLAKE2b │ 22000       │ 13    │ reference impl. │
                           │BLAKE2b │ 19000       │ 11    │ libgcrypt/AVX2  │
                           │BLAKE2b │ 19000       │ 11    │ libsodium/AVX2  │

       Many kernels are configured with SHA256 as built-in and not as a module.  The  accelerated
       versions  are  however  provided  by  the  modules and must be loaded explicitly (modprobe
       sha256)  before  mounting  the  filesystem  to  make  use  of  them.  You  can  check   in
       /sys/fs/btrfs/FSID/checksum  which  one  is  used. If you see sha256-generic, then you may
       want to unmount and mount the filesystem again, changing that on a mounted  filesystem  is
       not  possible.   Check  the  file /proc/crypto, when the implementation is built-in, you'd

          name         : sha256
          driver       : sha256-generic
          module       : kernel
          priority     : 100

       while accelerated implementation is e.g.

          name         : sha256
          driver       : sha256-avx2
          module       : sha256_ssse3
          priority     : 170


       Btrfs supports transparent file compression. There are three algorithms  available:  ZLIB,
       LZO  and ZSTD (since v4.14), with various levels.  The compression happens on the level of
       file extents and the algorithm is selected by file property, mount option or by  a  defrag
       command.   You  can  have  a  single  btrfs  mount  point  that  has  some  files that are
       uncompressed, some that are compressed with LZO, some with ZLIB, for instance (though  you
       may not want it that way, it is supported).

       Once  the  compression  is  set, all newly written data will be compressed, i.e.  existing
       data are untouched. Data are split into smaller chunks (128KiB) before compression to make
       random  rewrites  possible  without a high performance hit. Due to the increased number of
       extents the metadata consumption is higher. The chunks are compressed in parallel.

       The algorithms can be characterized as follows regarding the speed/ratio trade-offs:


              • slower, higher compression ratio

              • levels: 1 to 9, mapped directly, default level is 3

              • good backward compatibility


              • faster compression and decompression than ZLIB, worse compression ratio, designed
                to be fast

              • no levels

              • good backward compatibility


              • compression  comparable  to ZLIB with higher compression/decompression speeds and
                different ratio

              • levels: 1 to 15, mapped directly (higher levels are not available)

              • since 4.14, levels since 5.1

       The differences depend on the actual data set and cannot be expressed by a  single  number
       or  recommendation.  Higher  levels  consume more CPU time and may not bring a significant
       improvement, lower levels are close to real time.


       Typically the compression can be enabled on the whole filesystem, specified for the  mount
       point.  Note  that  the  compression mount options are shared among all mounts of the same
       filesystem, either bind mounts  or  subvolume  mounts.   Please  refer  to  section  MOUNT

          $ mount -o compress=zstd /dev/sdx /mnt

       This  will  enable the zstd algorithm on the default level (which is 3).  The level can be
       specified manually too like zstd:3. Higher levels compress better at  the  cost  of  time.
       This  in  turn  may  cause  increased write latency, low levels are suitable for real-time
       compression and on reasonably fast CPU don't cause noticeable performance drops.

          $ btrfs filesystem defrag -czstd file

       The command above will start defragmentation of the whole file and apply the  compression,
       regardless  of  the  mount  option.  (Note:  specifying level is not yet implemented). The
       compression algorithm is not persistent and applies only to the  defragmentation  command,
       for any other writes other compression settings apply.

       Persistent settings on a per-file basis can be set in two ways:

          $ chattr +c file
          $ btrfs property set file compression zstd

       The  first  command  is  using  legacy  interface  of  file attributes inherited from ext2
       filesystem and is not flexible, so by default the  zlib  compression  is  set.  The  other
       command  sets  a property on the file with the given algorithm.  (Note: setting level that
       way is not yet implemented.)


       The level support of ZLIB has been added in v4.14, LZO does not support levels (the kernel
       implementation provides only one), ZSTD level support has been added in v5.1.

       There  are  9  levels of ZLIB supported (1 to 9), mapping 1:1 from the mount option to the
       algorithm defined level. The default is  level  3,  which  provides  the  reasonably  good
       compression  ratio  and  is  still  reasonably fast. The difference in compression gain of
       levels 7, 8 and 9 is comparable but the higher levels take longer.

       The ZSTD support includes levels 1 to 15, a subset of full range of  what  ZSTD  provides.
       Levels  1-3  are  real-time, 4-8 slower with improved compression and 9-15 try even harder
       though the resulting size may not be significantly improved.

       Level 0 always maps to the default. The compression level does not affect compatibility.


       Files with already compressed data or with data that won't compress well with the CPU  and
       memory constraints of the kernel implementations are using a simple decision logic. If the
       first portion of data being compressed is not smaller than the original,  the  compression
       of  the  file is disabled -- unless the filesystem is mounted with compress-force. In that
       case compression will always be attempted on the file only to be later discarded. This  is
       not optimal and subject to optimizations and further development.

       If  a file is identified as incompressible, a flag is set (NOCOMPRESS) and it's sticky. On
       that file compression won't be performed unless forced. The flag can be also set by chattr
       +m (since e2fsprogs 1.46.2) or by properties with value no or none. Empty value will reset
       it to the default that's currently applicable on the mounted filesystem.

       There are two ways to detect incompressible data:

       • actual compression attempt - data are compressed, if the result  is  not  smaller,  it's
         discarded, so this depends on the algorithm and level

       • pre-compression heuristics - a quick statistical evaluation on the data is performed and
         based on the result either compression is performed or skipped, the  NOCOMPRESS  bit  is
         not  set  just  by  the  heuristic,  only  if the compression algorithm does not make an

          $ lsattr file
          ---------------------m file

       Using the forcing compression is not recommended, the heuristics are  supposed  to  decide
       that and compression algorithms internally detect incompressible data too.


       The  heuristics aim to do a few quick statistical tests on the compressed data in order to
       avoid probably costly compression that would  turn  out  to  be  inefficient.  Compression
       algorithms could have internal detection of incompressible data too but this leads to more
       overhead as the compression is done in another thread and has to write  the  data  anyway.
       The heuristic is read-only and can utilize cached memory.

       The  tests  performed  based  on  the  following:  data  sampling,  long  repeated pattern
       detection, byte frequency, Shannon entropy.


       Compression is done using the COW mechanism so it's incompatible with nodatacow. Direct IO
       works   on  compressed  files  but  will  fall  back  to  buffered  writes  and  leads  to
       recompression. Currently nodatasum and compression don't work together.

       The compression algorithms have been added over time so the version  compatibility  should
       be  also  considered,  together  with other tools that may access the compressed data like


       Btrfs has a sysfs interface to provide extra knobs.

       The top level path is /sys/fs/btrfs/, and the main directory layout is the following:

                  │Relative Path                │ Description              │ Version │
                  │features/                    │ All supported features   │ 3.14+   │
                  │<UUID>/                      │ Mounted fs UUID          │ 3.14+   │
                  │<UUID>/allocation/           │ Space allocation info    │ 3.14+   │
                  │<UUID>/features/             │ Features     of      the │ 3.14+   │
                  │                             │ filesystem               │         │
                  │<UUID>/devices/<DEVID>/      │ Symlink  to  each  block │ 5.6+    │
                  │                             │ device sysfs             │         │
                  │<UUID>/devinfo/<DEVID>/      │ Btrfs specific info  for │ 5.6+    │
                  │                             │ each device              │         │
                  │<UUID>/qgroups/              │ Global qgroup info       │ 5.9+    │
                  │<UUID>/qgroups/<LEVEL>_<ID>/ │ Info for each qgroup     │ 5.9+    │

       For /sys/fs/btrfs/features/ directory, each file means a supported feature for the current

       For  /sys/fs/btrfs/<UUID>/features/  directory, each file means an enabled feature for the
       mounted filesystem.

       The features shares the same name in section FILESYSTEM FEATURES.

       Files in /sys/fs/btrfs/<UUID>/ directory are:

              (RW, since: 5.19)

              Used space percentage of total device  space  to  start  auto  block  group  claim.
              Mostly for zoned devices.

              (RO, since: 5.5)

              The checksum used for the mounted filesystem.  This includes both the checksum type
              (see section CHECKSUM ALGORITHMS) and the implemented driver (mostly shows if  it's
              hardware accelerated).

              (RO, since: 3.16)

              The bytes alignment for clone and dedupe ioctls.

              (RW, since: 6.0)

              The  performance  statistics  for  btrfs  transaction  commit.   Mostly  for  debug

              Writing into this file will reset the maximum commit duration to the input value.

              (RO, since: 5.10)

              Shows  the  running  exclusive  operation.   Check  section  FILESYSTEM   EXCLUSIVE
              OPERATIONS for details.

              (RO, since: 5.11)

              Show the generation of the mounted filesystem.

       label  (RW, since: 3.14)

              Show the current label of the mounted filesystem.

              (RO, since: 5.0)

              Shows the metadata uuid of the mounted filesystem.  Check metadata_uuid feature for
              more details.

              (RO, since: 3.14)

              Show the nodesize of the mounted filesystem.

              (RW, since: 4.13)

              Shows the current quota override status.  0 means no quota override.  1 means quota
              override, quota can ignore the existing limit settings.

              (RW, since: 5.11)

              Shows  the  current  balance policy for reads.  Currently only "pid" (balance using
              pid value) is supported.

              (RO, since: 3.14)

              Shows the sectorsize of the mounted filesystem.

       Files and directories in /sys/fs/btrfs/<UUID>/allocations directory are:

              (RO, since: 3.14)

              The used bytes of the global reservation.

              (RO, since: 3.14)

              The total size of the global reservation.

       data/, metadata/ and system/ directories
              (RO, since: 5.14)

              Space info accounting for the 3 chunk types.  Mostly for debug purposes.

       Files in /sys/fs/btrfs/<UUID>/allocations/{data,metadata,system} directory are:

              (RW, since: 5.19)

              Reclaimable space percentage of block group's size (excluding permanently  unusable
              space) to reclaim the block group.  Can be used on regular or zoned devices.

              (RW, since: 6.0)

              Shows  the  chunk  size.  Can  be changed for data and metadata.  Cannot be set for
              zoned devices.

       Files in /sys/fs/btrfs/<UUID>/devinfo/<DEVID> directory are:

              (RO, since: 5.14)

              Shows all the history error numbers of the device.

       fsid:  (RO, since: 5.17)

              Shows the fsid which the device belongs to.  It can be different than the <UUID> if
              it's a seed device.

              (RO, since: 5.6)

              Shows whether we have found the device.  Should always be 1, as if this turns to 0,
              the <DEVID> directory would get removed automatically.

              (RO, since: 5.6)

              Shows whether the device is missing.

              (RO, since: 5.6)

              Shows whether the device is the replace target.  If no dev-replace is running, this
              value should be 0.

              (RW, since: 5.14)

              Shows  the  scrub  speed  limit  for  this device. The unit is Bytes/s.  0 means no

              (RO, since: 5.6)

              Show if the device is writeable.

       Files in /sys/fs/btrfs/<UUID>/qgroups/ directory are:

              (RO, since: 6.1)

              Shows if qgroup is enabled.  Also, if qgroup is  disabled,  the  qgroups  directory
              would be removed automatically.

              (RO, since: 6.1)

              Shows  if  the  qgroup  numbers  are  inconsistent.  If 1, it's recommended to do a
              qgroup rescan.

              (RW, since: 6.1)

              Shows the subtree drop threshold to automatically mark qgroup inconsistent.

              When dropping large subvolumes with qgroup enabled, there would be a huge load  for
              qgroup  accounting.   If  we  have a subtree whose level is larger than or equal to
              this value, we will not trigger qgroup account at all, but mark qgroup inconsistent
              to avoid the huge workload.

              Default value is 8, where no subtree drop can trigger qgroup.

              Lower  value  can  reduce  qgroup  workload,  at the cost of extra qgroup rescan to
              re-calculate the numbers.

       Files in /sys/fs/btrfs/<UUID>/<LEVEL>_<ID>/ directory are:

              (RO, since: 5.9)

              Shows the exclusively owned bytes of the qgroup.

              (RO, since: 5.9)

              Shows the numeric value of the limit flags.  If 0, means no limit implied.

              (RO, since: 5.9)

              Shows the limits on exclusively owned bytes.

              (RO, since: 5.9)

              Shows the limits on referenced bytes.

              (RO, since: 5.9)

              Shows the referenced bytes of the qgroup.

              (RO, since: 5.9)

              Shows the reserved bytes for data.

              (RO, since: 5.9)

              Shows the reserved bytes for per transaction metadata.

              (RO, since: 5.9)

              Shows the reserved bytes for preallocated metadata.


       There are several operations that affect  the  whole  filesystem  and  cannot  be  run  in
       parallel. Attempt to start one while another is running will fail (see exceptions below).

       Since   kernel   5.10   the   currently   running   operation   can   be   obtained   from
       /sys/fs/UUID/exclusive_operation with following values and operations:

       • balance

       • balance paused (since 5.17)

       • device add

       • device delete

       • device replace

       • resize

       • swapfile activate

       • none

       Enqueuing is supported for several btrfs subcommands so they can be started  at  once  and
       then serialized.

       There's  an exception when a paused balance allows to start a device add operation as they
       don't really collide and this can be used to add more space for the balance to finish.


       maximum file name length

              This limit is imposed by Linux VFS, the structures of BTRFS could store larger file

       maximum symlink target length
              depends  on  the nodesize value, for 4KiB 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,  i.e.  the  path  name  components  can
              exceed the limits (NAME_MAX), 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

              Each subvolume is an independent namespace of inodes and thus their numbers, so the
              limit is per subvolume, not for the whole filesystem.

       inode numbers
              minimum  number:  256 (for subvolumes), regular files and directories: 257, maximum
              number: (2:sup:64 - 256)

              The inode numbers that can be assigned to user created files  are  from  the  whole
              64bit  space  except  first  256  and  last 256 in that range that are reserved for
              internal b-tree identifiers.

       maximum file length
              inherent limit of BTRFS is 264 (16 EiB) but the practical limit of Linux VFS is 263
              (8 EiB)

       maximum number of subvolumes
              the  subvolume  ids can go up to 248 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). The range is not the full 64bit range because of qgroups
              that use the upper 16 bits for another purposes.

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

       minimum filesystem size
              the  minimal size of each device depends on the mixed-bg feature, without that (the
              default) it's about 109MiB, with mixed-bg it's is 16MiB


       GRUB2 ( has the most advanced support  of  booting  from
       BTRFS with respect to features.

       U-boot ( has decent support for booting but not all BTRFS
       features are implemented, check the documentation.

       EXTLINUX (from the project) has limited support for  BTRFS  boot  and
       hasn't been updated for for a long time so is not recommended as bootloader.

       In  general,  the  first  1MiB  on  each  device  is  unused with the exception of primary
       superblock that is on the offset 64KiB and spans 4KiB. The rest  can  be  freely  used  by
       bootloaders  or for other system information. Note that booting from a filesystem on zoned
       device is not supported.


       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

       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.  This
              attribute cannot be set with 'm' at the same time.

       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.

                 Due  to  implementation  limitations,  this  flag can be set/unset only on empty

       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

       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)

       m      no compression, permanently turn off compression on the given file. Any compression
              mount options will not affect this file. (chattr support added in 1.46.2)

              When set on a directory, all newly created files will inherit this attribute.  This
              attribute cannot be set with c at the same time.

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

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

       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 attribute S

       A      no atime updates, same as the attribute

       d      no dump, same as the attribute


       Since  version  5.12 btrfs supports so called zoned mode. This is a special on-disk format
       and allocation/write strategy that's friendly to zoned devices.  In  short,  a  device  is
       partitioned  into  fixed-size zones and each zone can be updated by append-only manner, or
       reset. As btrfs has no fixed data structures, except the super blocks, the zoned mode only
       requires  block  placement  that  follows  the device constraints. You can learn about the
       whole architecture at .

       The devices are also called SMR/ZBC/ZNS, in host-managed mode. Note that there are devices
       that  appear  as  non-zoned  but  actually are, this is drive-managed and using zoned mode
       won't help.

       The zone size depends on the device, typical sizes are 256MiB or 1GiB. In general it  must
       be a power of two. Emulated zoned devices like null_blk allow to set various zone sizes.

   Requirements, limitations
       • all devices must have the same zone size

       • maximum zone size is 8GiB

       • minimum zone size is 4MiB

       • mixing  zoned  and non-zoned devices is possible, the zone writes are emulated, but this
         is namely for testing


         the super block is handled in a special way and is at  different  locations  than  on  a
         non-zoned filesystem:

                • primary: 0B (and the next two zones)

                • secondary: 512GiB (and the next two zones)

                • tertiary: 4TiB (4096GiB, and the next two zones)

   Incompatible features
       The  main constraint of the zoned devices is lack of in-place update of the data.  This is
       inherently incompatible with some features:

       • NODATACOW - overwrite in-place, cannot create such files

       • fallocate - preallocating space for in-place first write

       • mixed-bg - unordered writes to data and metadata, fixing that means using separate  data
         and metadata block groups

       • booting - the zone at offset 0 contains superblock, resetting the zone would destroy the
         bootloader data

       Initial support lacks some features but they're planned:

       • only single profile is supported

       • fstrim - due to dependency on free space cache v1

   Super block
       As said above, super block is handled in a special way. In order  to  be  crash  safe,  at
       least  one  zone in a known location must contain a valid superblock.  This is implemented
       as a ring buffer in two consecutive zones, starting from  known  offsets  0B,  512GiB  and

       The  values  are  different than on non-zoned devices. Each new super block is appended to
       the end of the zone, once it's filled, the zone is reset and writes continue to  the  next
       one.  Looking  up the latest super block needs to read offsets of both zones and determine
       the last written version.

       The amount of space reserved for super block depends on the zone size. The  secondary  and
       tertiary  copies  are  at distant offsets as the capacity of the devices is expected to be
       large, tens of terabytes. Maximum zone size supported is 8GiB, which would mean that  e.g.
       offset 0-16GiB would be reserved just for the super block on a hypothetical device of that
       zone size. This is wasteful but required to guarantee crash safety.


       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

       • scan  devices  for  btrfs  filesystem  (i.e.  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

       • get the supported features (can be also found under /sys/fs/btrfs/features)

       The device is created when btrfs  is  initialized,  either  as  a  module  or  a  built-in
       functionality  and makes sense only in connection with that. Running e.g. mkfs without the
       module loaded will not register the device and will probably warn about that.

       In rare cases when the module is loaded  but  the  device  is  not  present  (most  likely
       accidentally deleted), it's possible to recreate it by

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

       or (since 5.11) by a convenience command

          # btrfs rescue create-control-device

       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 btrfs device scan.


       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

       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.

          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 RAID56, 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)


       The  COW  mechanism  and  multiple  devices  under one hood enable an interesting concept,
       called a seeding device: extending a read-only filesystem on a device with another  device
       that  captures  all  writes. For example imagine an immutable golden image of an operating
       system enhanced with another device that allows to use the data from the golden image  and
       normal  operation.   This idea originated on CD-ROMs with base OS and allowing to use them
       for live systems, but this became  obsolete.  There  are  technologies  providing  similar
       functionality, like unionmount, overlayfs or qcow2 image snapshot.

       The seeding device starts as a normal filesystem, once the contents is ready, btrfstune -S
       1 is used to flag it as a seeding device. Mounting such device will not allow any  writes,
       except  adding  a new device by btrfs device add.  Then the filesystem can be remounted as

       Given that the filesystem on the seeding device is always recognized as read-only, it  can
       be  used  to  seed multiple filesystems from one device at the same time. The UUID that is
       normally attached to a device is automatically changed to a random UUID on each mount.

       Once the seeding device is mounted,  it  needs  the  writable  device.  After  adding  it,
       something  like  remount  -o remount,rw /path makes the filesystem at /path ready for use.
       The simplest use case is to throw away all  changes  by  unmounting  the  filesystem  when

       Alternatively,  deleting  the seeding device from the filesystem can turn it into a normal
       filesystem, provided that the writable device can also  contain  all  the  data  from  the
       seeding device.

       The  seeding  device  flag  can  be  cleared again by btrfstune -f -S 0, e.g.  allowing to
       update with newer data but please note that this will invalidate all existing  filesystems
       that  use  this  particular seeding device. This works for some use cases, not for others,
       and the forcing flag to the command is mandatory to avoid accidental mistakes.

       Example how to create and use one seeding device:

          # mkfs.btrfs /dev/sda
          # mount /dev/sda /mnt/mnt1
          ... fill mnt1 with data
          # umount /mnt/mnt1

          # btrfstune -S 1 /dev/sda

          # mount /dev/sda /mnt/mnt1
          # btrfs device add /dev/sdb /mnt/mnt1
          # mount -o remount,rw /mnt/mnt1
          ... /mnt/mnt1 is now writable

       Now /mnt/mnt1 can be used normally. The device  /dev/sda  can  be  mounted  again  with  a
       another writable device:

          # mount /dev/sda /mnt/mnt2
          # btrfs device add /dev/sdc /mnt/mnt2
          # mount -o remount,rw /mnt/mnt2
          ... /mnt/mnt2 is now writable

       The  writable  device  (/dev/sdb)  can  be  decoupled  from  the  seeding  device and used

          # btrfs device delete /dev/sda /mnt/mnt1

       As the contents originated in the seeding device, it's possible  to  turn  /dev/sdb  to  a
       seeding device again and repeat the whole process.

       A few things to note:

       • it's recommended to use only single device for the seeding device, it works for multiple
         devices but the single profile must be used in order to make the seeding device deletion

       • block group profiles single and dup support the use cases above

       • the label is copied from the seeding device and can be changed by btrfs filesystem label

       • each new mount of the seeding device gets a new random UUID

   Chained seeding devices
       Though  it's  not  recommended  and  is  rather an obscure and untested use case, chaining
       seeding devices is possible. In the first example, the writable  device  /dev/sdb  can  be
       turned  onto  another  seeding  device  again,  depending  on the unchanged seeding device
       /dev/sda. Then using /dev/sdb as the primary  seeding  device  it  can  be  extended  with
       another  writable  device,  say  /dev/sdd,  and  it  continues  as before as a simple tree
       structure on devices.

          # mkfs.btrfs /dev/sda
          # mount /dev/sda /mnt/mnt1
          ... fill mnt1 with data
          # umount /mnt/mnt1

          # btrfstune -S 1 /dev/sda

          # mount /dev/sda /mnt/mnt1
          # btrfs device add /dev/sdb /mnt/mnt1
          # mount -o remount,rw /mnt/mnt1
          ... /mnt/mnt1 is now writable
          # umount /mnt/mnt1

          # btrfstune -S 1 /dev/sdb

          # mount /dev/sdb /mnt/mnt1
          # btrfs device add /dev/sdc /mnt
          # mount -o remount,rw /mnt/mnt1
          ... /mnt/mnt1 is now writable
          # umount /mnt/mnt1

       As a result we have:

       • sda is a single seeding device, with its initial contents

       • sdb is a seeding device but requires sda, the contents are from the  time  when  sdb  is
         made seeding, i.e. contents of sda with any later changes

       • sdc  last  writable,  can  be made a seeding one the same way as was sdb, preserving its
         contents and depending on sda and sdb

       As long as the seeding devices are unmodified and available, they can  be  used  to  start
       another branch.


       The  RAID56  feature  provides  striping  and  parity  over  several  devices, same as the
       traditional RAID5/6. There are some implementation and design deficiencies  that  make  it
       unreliable  for  some  corner cases and the feature should not be used in production, only
       for evaluation or testing.  The power failure safety for metadata with RAID56 is not 100%.

       Do not use raid5 nor raid6 for metadata. Use raid1 or raid1c3 respectively.

       The substitute profiles provide the same guarantees against loss of 1 or 2 devices, and in
       some  respect can be an improvement.  Recovering from one missing device will only need to
       access the remaining 1st or 2nd copy, that in general may be stored on some other  devices
       due  to  the way RAID1 works on btrfs, unlike on a striped profile (similar to raid0) that
       would need all devices all the time.

       The space allocation pattern and consumption is different (e.g. on N devices):  for  raid5
       as an example, a 1GiB chunk is reserved on each device, while with raid1 there's each 1GiB
       chunk stored on 2 devices. The consumption of each 1GiB of used metadata is then N *  1GiB
       for  vs  2  *  1GiB. Using raid1 is also more convenient for balancing/converting to other
       profile due to lower requirement on the available chunk space.

   Missing/incomplete support
       When RAID56 is on the same filesystem with different raid profiles, the space reporting is
       inaccurate,  e.g.  df,  btrfs filesystem df or btrfs filesystem usage. When there's only a
       one profile per block group type (e.g. RAID5 for data) the reporting is accurate.

       When scrub is started on a RAID56 filesystem, it's started on all devices that degrade the
       performance.  The  workaround  is  to  start it on each device separately. Due to that the
       device stats may not match the actual state and some errors might  get  reported  multiple

       The  write  hole  problem. An unclean shutdown could leave a partially written stripe in a
       state where the some stripe ranges and the parity are from the old  writes  and  some  are
       new.  The  information  which  is  which is not tracked. Write journal is not implemented.
       Alternatively a full read-modify-write would make  sure  that  a  full  stripe  is  always
       written, avoiding the write hole completely, but performance in that case turned out to be
       too bad for use.

       The striping happens on all available devices (at the time the chunks were allocated),  so
       in  case  a  new  device  is  added it may not be utilized immediately and would require a
       rebalance. A fixed configured stripe width is not implemented.


   Storage model
       A storage model is a model that captures key physical aspects of data structure in a  data
       store. A filesystem is the logical structure organizing data on top of the storage device.

       The  filesystem assumes several features or limitations of the storage device and utilizes
       them or applies measures to guarantee reliability. BTRFS in particular is based on  a  COW
       (copy  on write) mode of writing, i.e. not updating data in place but rather writing a new
       copy to a different location and then atomically switching the pointers.

       In an ideal world, the device does what it promises. The filesystem assumes that this  may
       not  be true so additional mechanisms are applied to either detect misbehaving hardware or
       get valid data by other means. The devices may (and do)  apply  their  own  detection  and
       repair mechanisms but we won't assume any.

       The  following  assumptions  about  storage  devices are considered (sorted by importance,
       numbers are for further reference):

       1. atomicity of reads and writes of blocks/sectors (the smallest unit of data  the  device
          presents to the upper layers)

       2. there's  a  flush command that instructs the device to forcibly order writes before and
          after the command;  alternatively  there's  a  barrier  command  that  facilitates  the
          ordering but may not flush the data

       3. data  sent to write to a given device offset will be written without further changes to
          the data and to the offset

       4. writes can be reordered by the  device,  unless  explicitly  serialized  by  the  flush

       5. reads and writes can be freely reordered and interleaved

       The  consistency  model of BTRFS builds on these assumptions. The logical data updates are
       grouped, into a generation, written on the device, serialized by  the  flush  command  and
       then  the  super block is written ending the generation.  All logical links among metadata
       comprising a consistent view of the data may not cross the generation boundary.

   When things go wrong
       No or partial atomicity of block reads/writes (1)Problem: a partial block contents is written (torn write), e.g. due to a power glitch or
         other electronics failure during the read/write

       • Detection: checksum mismatch on read

       • Repair: use another copy or rebuild from multiple blocks using some encoding scheme

       The flush command does not flush (2)

       This  is perhaps the most serious problem and impossible to mitigate by filesystem without
       limitations and design restrictions. What could happen in the worst case  is  that  writes
       from  one generation bleed to another one, while still letting the filesystem consider the
       generations isolated. Crash at any point would leave data on the device in an inconsistent
       state  without  any  hint  what  exactly got written, what is missing and leading to stale
       metadata link information.

       Devices usually honor the flush command, but  for  performance  reasons  may  do  internal
       caching,  where  the  flushed  data are not yet persistently stored. A power failure could
       lead to a similar scenario as above, although it's less likely that later writes would  be
       written  before  the  cached ones. This is beyond what a filesystem can take into account.
       Devices or controllers are usually equipped with batteries  or  capacitors  to  write  the
       cache contents even after power is cut. (Battery backed write cache)

       Data get silently changed on write (3)

       Such  thing  should not happen frequently, but still can happen spuriously due the complex
       internal workings of devices or physical effects of the storage media itself.

       • Problem: while the data are written atomically, the contents get changed

       • Detection: checksum mismatch on read

       • Repair: use another copy or rebuild from multiple blocks using some encoding scheme

       Data get silently written to another offset (3)

       This would be another serious problem  as  the  filesystem  has  no  information  when  it
       happens. For that reason the measures have to be done ahead of time.  This problem is also
       commonly called ghost write.

       The metadata blocks have the checksum embedded in the blocks, so a  correct  atomic  write
       would  not corrupt the checksum. It's likely that after reading such block the data inside
       would not be consistent with the rest. To rule that out there's embedded block  number  in
       the  metadata  block.  It's  the  logical  block  number  because this is what the logical
       structure expects and verifies.

       The following is  based  on  information  publicly  available,  user  feedback,  community
       discussions  or  bug report analyses. It's not complete and further research is encouraged
       when in doubt.

   Main memory
       The data structures and raw data blocks are temporarily stored in computer  memory  before
       they get written to the device. It is critical that memory is reliable because even simple
       bit flips can have vast consequences and lead to  damaged  structures,  not  only  in  the
       filesystem but in the whole operating system.

       Based  on  experience  in  the  community, memory bit flips are more common than one would
       think. When it happens, it's reported by the tree-checker or by a checksum mismatch  after
       reading blocks. There are some very obvious instances of bit flips that happen, e.g. in an
       ordered sequence of keys in metadata blocks. We can easily infer from the other data  what
       values  get damaged and how. However, fixing that is not straightforward and would require
       cross-referencing data from the entire filesystem to see the scope.

       If available, ECC memory should lower the chances of bit flips, but this type of memory is
       not  available  in  all cases. A memory test should be performed in case there's a visible
       bit flip pattern, though this may not detect a faulty memory  module  because  the  actual
       load of the system could be the factor making the problems appear. In recent years attacks
       on how the memory modules operate have been demonstrated  (rowhammer)  achieving  specific
       bits  to  be  flipped.   While  these  were targeted, this shows that a series of reads or
       writes can affect unrelated parts of memory.

       Further reading:


       What to do:

       • run memtest, note that sometimes memory errors happen only  when  the  system  is  under
         heavy load that the default memtest cannot trigger

       • memory  errors  may  appear as filesystem going read-only due to "pre write" check, that
         verify meta data before they get written but fail some basic consistency checks

   Direct memory access (DMA)
       Another class of errors is related to DMA  (direct  memory  access)  performed  by  device
       drivers.  While  this could be considered a software error, the data transfers that happen
       without CPU assistance may accidentally corrupt other pages. Storage devices  utilize  DMA
       for  performance  reasons,  the  filesystem  structures and data pages are passed back and
       forth, making errors possible in case page life time is not properly tracked.

       There are lots of quirks (device-specific workarounds) in Linux kernel drivers  (regarding
       not  only  DMA) that are added when found. The quirks may avoid specific errors or disable
       some features to avoid worse problems.

       What to do:

       • use up-to-date kernel (recent releases or maintained long term support versions)

       • as this may be caused by faulty drivers, keep the systems up-to-date

   Rotational disks (HDD)
       Rotational HDDs typically fail at the level of individual sectors or small clusters.  Read
       failures are caught on the levels below the filesystem and are returned to the user as EIO
       - Input/output error. Reading the blocks repeatedly may return the  data  eventually,  but
       this  is  better  done  by  specialized tools and filesystem takes the result of the lower
       layers. Rewriting the sectors may trigger internal remapping but this inevitably leads  to
       data loss.

       Disk  firmware  is technically software but from the filesystem perspective is part of the
       hardware. IO requests are processed,  and  caching  or  various  other  optimizations  are
       performed,  which  may  lead  to bugs under high load or unexpected physical conditions or
       unsupported use cases.

       Disks are connected by cables with two ends, both of which can  cause  problems  when  not
       attached properly. Data transfers are protected by checksums and the lower layers try hard
       to transfer the data correctly or not at all. The errors from badly-connecting cables  may
       manifest  as  large  amount  of  failed  read  or write requests, or as short error bursts
       depending on physical conditions.

       What to do:

       • check smartctl for potential issues

   Solid state drives (SSD)
       The mechanism of information storage is different from HDDs and this affects  the  failure
       mode  as well. The data are stored in cells grouped in large blocks with limited number of
       resets and other write constraints. The firmware tries to  avoid  unnecessary  resets  and
       performs  optimizations  to  maximize the storage media lifetime. The known techniques are
       deduplication (blocks with same fingerprint/hash  are  mapped  to  same  physical  block),
       compression  or internal remapping and garbage collection of used memory cells. Due to the
       additional processing there are measures to verity the data e.g. by ECC codes.

       The observations of failing SSDs show that the whole electronic fails at once or affects a
       lot of data (e.g. stored on one chip). Recovering such data may need specialized equipment
       and reading data repeatedly does not help as it's possible with HDDs.

       There are several technologies of the memory  cells  with  different  characteristics  and
       price.  The  lifetime  is  directly  affected  by  the type and frequency of data written.
       Writing "too much" distinct data (e.g. encrypted) may render  the  internal  deduplication
       ineffective and lead to a lot of rewrites and increased wear of the memory cells.

       There  are  several technologies and manufacturers so it's hard to describe them but there
       are some that exhibit similar behaviour:

       • expensive SSD will use more durable memory cells and is optimized  for  reliability  and
         high load

       • cheap  SSD  is projected for a lower load ("desktop user") and is optimized for cost, it
         may employ the optimizations and/or extended error reporting partially or not at all

       It's not possible to reliably determine the expected lifetime of an SSD  due  to  lack  of
       information about how it works or due to lack of reliable stats provided by the device.

       Metadata  writes tend to be the biggest component of lifetime writes to a SSD, so there is
       some value in reducing them. Depending on the device class (high end/low end) the features
       like DUP block group profiles may affect the reliability in both ways:

       • high  end  are  typically  more reliable and using single for data and metadata could be
         suitable to reduce device wear

       • low end could lack ability to  identify  errors  so  an  additional  redundancy  at  the
         filesystem level (checksums, DUP) could help

       Only users who consume 50 to 100% of the SSD's actual lifetime writes need to be concerned
       by the write amplification of btrfs DUP metadata. Most users will be far below 50% of  the
       actual lifetime, or will write the drive to death and discover how many writes 100% of the
       actual lifetime was. SSD firmware often  adds  its  own  write  multipliers  that  can  be
       arbitrary  and  unpredictable  and  dependent  on  application  behavior,  and  these will
       typically have far greater effect on SSD lifespan than DUP metadata.  It's  more  or  less
       impossible  to  predict  when  a SSD will run out of lifetime writes to within a factor of
       two, so it's hard to justify wear reduction as a benefit.

       Further reading:


       What to do:

       • run smartctl or self-tests to look for potential issues

       • keep the firmware up-to-date

   NVM express, non-volatile memory (NVMe)
       NVMe is a type of persistent memory usually connected over a system bus (PCIe) or  similar
       interface  and  the  speeds  are  an  order  of  magnitude  faster than SSD.  It is also a
       non-rotating type of storage, and is not typically connected by a cable. It's not  a  SCSI
       type device either but rather a complete specification for logical device interface.

       In  a  way  the errors could be compared to a combination of SSD class and regular memory.
       Errors may exhibit as random bit flips or IO failures.  There  are  tools  to  access  the
       internal log (nvme log and nvme-cli) for a more detailed analysis.

       There  are  separate  error detection and correction steps performed e.g. on the bus level
       and in most cases never making in to the filesystem level. Once this happens it could mean
       there's  some  systematic error like overheating or bad physical connection of the device.
       You may want to run self-tests (using smartctl).


   Drive firmware
       Firmware is technically still software but embedded into the hardware. As all software has
       bugs,  so  does  firmware.  Storage devices can update the firmware and fix known bugs. In
       some cases the it's possible to avoid certain bugs by quirks (device-specific workarounds)
       in Linux kernel.

       A  faulty  firmware  can cause wide range of corruptions from small and localized to large
       affecting lots of data. Self-repair capabilities may not be sufficient.

       What to do:

       • check for firmware updates in case there are known problems, note that updating firmware
         can be risky on itself

       • use up-to-date kernel (recent releases or maintained long term support versions)

   SD flash cards
       There  are  a lot of devices with low power consumption and thus using storage media based
       on low power consumption too, typically flash memory  stored  on  a  chip  enclosed  in  a
       detachable  card  package. An improperly inserted card may be damaged by electrical spikes
       when the device is turned on or off. The  chips  storing  data  in  turn  may  be  damaged
       permanently.  All types of flash memory have a limited number of rewrites, so the data are
       internally translated by FTL (flash translation layer). This is  implemented  in  firmware
       (technically a software) and prone to bugs that manifest as hardware errors.

       Adding  redundancy  like  using  DUP  profiles for both data and metadata can help in some
       cases but a full backup might be the best option once problems appear  and  replacing  the
       card could be required as well.

   Hardware as the main source of filesystem corruptions
       If  you use unreliable hardware and don't know about that, don't blame the filesystem when
       it tells you.


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