Provided by: btrfs-progs_4.16.1-2ubuntu1_amd64 bug

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.

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

       --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 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 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.

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)