btrfs-check(8) ============== NAME ---- btrfs-check - check or repair an unmounted 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. 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 :: use the given offset 'bytenr' for the chunk tree root -E|--subvol-extents :: 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 :: 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 '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)