btrfs-progs/Documentation/btrfs-check.asciidoc

btrfs-check(8)
==============

NAME
----
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, 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.

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 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

-p|--progress::
indicate progress at various checking phases

-Q|--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 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.

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)