From btrfs Wiki
Revision as of 20:35, 29 September 2017 by Kdave (Talk | contribs)

Jump to: navigation, search


btrfs-check(8) manual page

Note: manual pages are located at read-the-docs site, please update your links.


btrfs-check - check or repair a btrfs filesystem


btrfs check [options] <device>


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 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, 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.



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.


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 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


indicate progress at various checking phases


verify qgroup accounting and compare against filesystem accounting

-r|--tree-root <bytenr>

use the given offset bytenr for the tree root


(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.



enable the repair mode and attempt to fix problems where possible


create a new checksum tree and recalculate checksums in all files

Note: Do not blindly use this option to fix checksum mismatch problems.


build the extent tree from scratch

Note: Do not use unless you know what you’re doing.


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.


allow to 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.


btrfs check returns a zero exit status if it succeeds. Non zero is returned in case of failure.


btrfs is part of btrfs-progs. Please refer to the btrfs wiki for further details.


mkfs.btrfs(8), btrfs-scrub(8), btrfs-rescue(8)

Personal tools