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.
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 filesystem
corruption. Eg. 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
use the first valid set of backup roots stored in the superblock
This can be combined with --super if some of the superblocks are
verify checksums of data blocks
This expects that the filesystem is otherwise OK, and is
basically and offline scrub but does not repair data from spare
use the given offset bytenr for the chunk tree root
show extent state for the given subvolume
indicate progress at various checking phases
verify qgroup accounting and compare against filesystem
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
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.
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
destroys 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
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 documentation have shown to
create a new checksum tree and recalculate checksums in all files
Do not blindly use this option to fix checksum mismatch
build the extent tree from scratch
Do not use unless you know what you’re doing.
select mode of operation regarding memory and IO
The MODE can be one of:
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 possible workaround is to
export the block device over network to a machine with enough
This mode is supposed to address the high memory consumption
at the cost of increased IO when it needs to re-read blocks.
This may increase run time.
Notelowmem mode does not work with --repair yet, and is still
allow 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.
This option also skips the delay and warning in the repair mode
This page is part of the btrfs-progs (btrfs filesystem tools)
project. Information about the project can be found at
If you have a bug report for this manual page, see
This page was obtained from the project's upstream Git repository
on 2020-08-13. (At that time, the date of the most recent commit
that was found in the repository was 2020-07-02.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
Btrfs v4.6.1 05/16/2020 BTRFS-CHECK(8)