# man > BTRFS-CHECK(8) [_BTRFS-CHECK_(8)](https://www.chedong.com/phpMan.php/man/BTRFS-CHECK/8/markdown) BTRFS [_BTRFS-CHECK_(8)](https://www.chedong.com/phpMan.php/man/BTRFS-CHECK/8/markdown) ## NAME btrfs-check - check or repair a btrfs filesystem ## SYNOPSIS **btrfs check **[options] ## 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 filesys‐ tem corruption. E.g. 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 an offline _scrub_ that does not repair data from spare copies. **--chunk-root _**_ use the given offset _bytenr_ for the chunk tree root ### -E|--subvol-extents show extent state for the given subvolume **--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 possi‐ ble 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 in‐ creased IO when it needs to re-read blocks. This may increase run time. ### -p|--progress indicate progress at various checking phases ### -Q|--qgroup-report verify qgroup accounting and compare against filesystem accounting ### -r|--tree-root 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 use Nth superblock copy, valid values are 0, 1 or 2 if the respective superblock off‐ set 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 remove the free space cache of the given version See also the _clear_cache_ mount option. **WARNING:** This option is deprecated, please use _btrfs_ _rescue_ _clear-space-cache_ instead, this option would be removed in the future eventually. ## 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 documen‐ tation have shown to be insufficient. ### --init-csum-tree create a new checksum tree and recalculate checksums in all files **WARNING:** Do not blindly use this option to fix checksum mismatch problems. ### --init-extent-tree build the extent tree from scratch **WARNING:** Do not use unless you know what you're doing. ### --force allow work on a mounted filesystem and skip mount checks. Note that this should work fine on a quiescent or read-only mounted filesystem but may crash if the device is changed externally, e.g. by the kernel module. **NOTE:** It is possible to run with --repair but on a mounted filesystem that will most likely lead to a corruption unless the filesystem is in a quiescent state which may not be possible to guarantee. This option also skips the delay and warning in the repair mode (see _--repair_). ## DEPRECATED OR MOVED OPTIONS ### --clear-ino-cache (removed: 6.7) remove leftover items pertaining to the deprecated _inode_ _cache_ feature, please use _btrfs_ _rescue_ _clear-ino-cache_ instead ## EXIT STATUS **btrfs check **returns a zero exit status if it succeeds. Non zero is returned in case of fail‐ ure. ## AVAILABILITY **btrfs **is part of btrfs-progs. Please refer to the documentation at _. ## SEE ALSO [_mkfs.btrfs(8)](https://www.chedong.com/phpMan.php/man/mkfs.btrfs/8/markdown)_, [_btrfs-scrub(8)](https://www.chedong.com/phpMan.php/man/btrfs-scrub/8/markdown)_, [_btrfs-rescue(8)](https://www.chedong.com/phpMan.php/man/btrfs-rescue/8/markdown)_ 6.6.3 Mar 31, 2024 [_BTRFS-CHECK_(8)](https://www.chedong.com/phpMan.php/man/BTRFS-CHECK/8/markdown)