btrfs-check(8) — Linux manual page


BTRFS-CHECK(8)                Btrfs Manual                BTRFS-CHECK(8)

NAME         top

       btrfs-check - check or repair a btrfs filesystem

SYNOPSIS         top

       btrfs check [options] <device>

DESCRIPTION         top

       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

       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.


           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. Eg. 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.


           use the first valid set of backup roots stored in the

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

           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

           indicate progress at various checking phases

           verify qgroup accounting and compare against filesystem

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

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

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

           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.

           remove leftover items pertaining to the deprecated inode map


           enable the repair mode and attempt to fix problems where

               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

           create a new checksum tree and recalculate checksums in all

               Do not blindly use this option to fix checksum mismatch

           build the extent tree from scratch

               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:

               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.

               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.

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

           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, eg. 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         top

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

AVAILABILITY         top

       btrfs is part of btrfs-progs. Please refer to the btrfs wiki for further details.

SEE ALSO         top

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

COLOPHON         top

       This page is part of the btrfs-progs (btrfs filesystem tools)
       project.  Information about the project can be found at 
       If you have a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       on 2024-06-14.  (At that time, the date of the most recent commit
       that was found in the repository was 2024-05-02.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to

Btrfs v5.16.1                  02/06/2022                 BTRFS-CHECK(8)

Pages that refer to this page: btrfs(8)btrfs-rescue(8)btrfs-restore(8)fsck.btrfs(8)