btrfs-check man page

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)

Referenced By

btrfs(8), btrfs-rescue(8), btrfs-restore(8), fsck.btrfs(8).

11/28/2016 Btrfs v4.8.4 Btrfs Manual