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

NAME         top

       btrfs-check - check or repair an unmounted 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. The filesystem must
       be unmounted.

       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 by a developer, an
           experienced user or accept the fact that fsck cannot possibly fix
           all sorts of damage that could happen to a filesystem because of
           software and hardware bugs.

       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.


           use the first valid set of backup roots stored in the superblock

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

           verify checksums of data blocks

           This expects that the filesystem is otherwise OK, so this is
           basically and offline scrub but does not repair data from spare

       --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 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 does
           destroy 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.


           enable the repair mode and attempt to fix problems where possible

           create a new checksum tree and recalculate checksums in all files

               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.

           select mode of operation regarding memory and IO

           The MODE can be one of original and lowmem. The original mode is
           mostly unoptimized regarding memory consumption and can lead to
           out-of-memory conditions on large filesystems. The possible
           workaround is to export the block device over network to a
           machine with enough memory. The low memory mode is supposed to
           address the memory consumption, at the cost of increased IO when
           it needs to re-read blocks when needed. This may increase run

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

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 
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Btrfs v4.6.1                     03/12/2017                   BTRFS-CHECK(8)