NAME

       btrfs-check - check or repair a btrfs filesystem

SYNOPSIS

       btrfs check [options] <device>

DESCRIPTION

       The  filesystem checker is used to verify structural integrity of a filesystem and attempt
       to repair it if requested.  It is recommended to unmount the filesystem prior  to  running
       the check, but it is possible to start checking a mounted filesystem (see --force).

       By default, btrfs check will not modify the device but you can reaffirm that by the option
       --readonly.

       btrfsck is an alias of btrfs check command and is now deprecated.

       WARNING:
          Do not use --repair unless you are advised to do so by a developer  or  an  experienced
          user, and then only after having accepted that no fsck successfully repair all types of
          filesystem corruption. E.g. some other software or hardware bugs can fatally  damage  a
          volume.

       The  structural integrity check verifies if internal filesystem objects or data structures
       satisfy the constraints, point to the right objects or are correctly connected together.

       There are several cross checks that can detect wrong reference counts of  shared  extents,
       backreferences, missing extents of inodes, directory and inode connectivity etc.

       The  amount  of  memory  required  can  be  high, depending on the size of the filesystem,
       similarly the run time. Check the modes that can also affect that.

SAFE OR ADVISORY OPTIONS

       -b|--backup
              use the first valid set of backup roots stored in the superblock

              This can be combined with --super if some of the superblocks are damaged.

       --check-data-csum
              verify checksums of data blocks

              This expects that the filesystem is otherwise OK, and is basically an offline scrub
              that does not repair data from spare copies.

       --chunk-root <bytenr>
              use the given offset bytenr for the chunk tree root

       -E|--subvol-extents <subvolid>
              show extent state for the given subvolume

       -p|--progress
              indicate progress at various checking phases

       -Q|--qgroup-report
              verify qgroup accounting and compare against filesystem accounting

       -r|--tree-root <bytenr>
              use the given offset 'bytenr' for the tree root

       --readonly
              (default)  run  in  read-only mode, this option exists to calm potential panic when
              users are going to run the checker

       -s|--super <N>
              use Nth superblock copy, valid values are 0, 1 or 2 if  the  respective  superblock
              offset is within the device size

              This  can  be  used  to  use  a  different  starting  point  if some of the primary
              superblock is damaged.

       --clear-space-cache v1|v2
              completely wipe all free space cache of given type

              For free space cache v1, the clear_cache kernel mount option only rebuilds the free
              space cache for block groups that are modified while the filesystem is mounted with
              that option. Thus, using this option with v1 makes it possible  to  actually  clear
              the entire free space cache.

              For  free  space  cache v2, the clear_cache kernel mount option destroys the entire
              free space cache. This option, with v2 provides an alternative method  of  clearing
              the free space cache that doesn't require mounting the filesystem.

       --clear-ino-cache
              remove leftover items pertaining to the deprecated inode map feature

DANGEROUS OPTIONS

       --repair
              enable the repair mode and attempt to fix problems where possible

              NOTE:
                 There's a warning and 10 second delay when this option is run without --force to
                 give users a chance to think  twice  before  running  repair,  the  warnings  in
                 documentation have shown to be insufficient

       --init-csum-tree
              create a new checksum tree and recalculate checksums in all files

              WARNING:
                 Do not blindly use this option to fix checksum mismatch problems.

       --init-extent-tree
              build the extent tree from scratch

              WARNING:
                 Do not use unless you know what you're doing.

       --mode <MODE>
              select mode of operation regarding memory and IO

              The MODE can be one of:

              original
                     The  metadata  are  read into memory and verified, thus the requirements are
                     high on large filesystems and can even  lead  to  out-of-memory  conditions.
                     The  possible  workaround  is  to  export the block device over network to a
                     machine with enough memory.

              lowmem This mode is supposed to address the high memory consumption at the cost  of
                     increased IO when it needs to re-read blocks.  This may increase run time.

              NOTE:
                 lowmem   mode  does  not  work  with  --repair  yet,  and  is  still  considered
                 experimental.

       --force
              allow work on a mounted filesystem. Note that this should work fine on a  quiescent
              or  read-only mounted filesystem but may crash if the device is changed externally,
              e.g. by the kernel module.  Repair without mount checks is not supported right now.

              This option also skips the delay and warning in the repair mode (see --repair).

EXIT STATUS

       btrfs check returns a zero exit status if it succeeds. Non zero is  returned  in  case  of
       failure.

AVAILABILITY

       btrfs    is    part    of   btrfs-progs.    Please   refer   to   the   documentation   at
       https://btrfs.readthedocs.io or wiki http://btrfs.wiki.kernel.org for further information.

SEE ALSO

       mkfs.btrfs(8), btrfs-scrub(8), btrfs-rescue(8)