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

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. 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.

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
           and offline scrub but 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 _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
           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.

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

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

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

               Note
               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, 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
       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 btrfs wiki
       http://btrfs.wiki.kernel.org for further details.

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

Btrfs v5.4.1                      01/09/2020                    BTRFS-CHECK(8)

NAME | SYNOPSIS | DESCRIPTION | SAFE OR ADVISORY OPTIONS | DANGEROUS OPTIONS | EXIT STATUS | AVAILABILITY | SEE ALSO