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

NAME
       btrfs-check - check or repair an unmounted 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. 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.
           Warning
           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, backrefrences, missing extents of inodes, directory
       and inode connectivity etc.
       The amount of memory required can be high, depending on the size of the
       filesystem, smililarly 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, so this is
           basically and offline scrub but does not repair data from spare
           coipes.
       --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
       --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 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.
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 consumpption 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.
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 v4.9.1                      08/06/2017                    BTRFS-CHECK(8)