6 btrfs-check - check or repair a btrfs filesystem
10 *btrfs check* [options] <device>
15 The filesystem checker is used to verify structural integrity of a filesystem
16 and attempt to repair it if requested. It is recommended to unmount the
17 filesystem prior to running the check, but it is possible to start checking a
18 mounted filesystem (see '--force').
20 By default, *btrfs check* will not modify the device but you can reaffirm that
21 by the option '--readonly'.
23 *btrfsck* is an alias of *btrfs check* command and is now deprecated.
25 WARNING: Do not use '--repair' unless you are advised to by a developer, an
26 experienced user or accept the fact that 'fsck' cannot possibly fix all sorts
27 of damage that could happen to a filesystem because of software and hardware
30 The structural integrity check verifies if internal filesystem objects or
31 data structures satisfy the constraints, point to the right objects or are
32 correctly connected together.
34 There are several cross checks that can detect wrong reference counts of shared
35 extents, backreferences, missing extents of inodes, directory and inode
38 The amount of memory required can be high, depending on the size of the
39 filesystem, similarly the run time.
41 SAFE OR ADVISORY OPTIONS
42 ------------------------
45 use the first valid set of backup roots stored in the superblock
47 This can be combined with '--super' if some of the superblocks are damaged.
50 verify checksums of data blocks
52 This expects that the filesystem is otherwise
53 OK, so this is basically and offline 'scrub' but does not repair data from
56 --chunk-root <bytenr>::
57 use the given offset 'bytenr' for the chunk tree root
59 -E|--subvol-extents <subvolid>::
60 show extent state for the given subvolume
63 indicate progress at various checking phases
66 verify qgroup accounting and compare against filesystem accounting
68 -r|--tree-root <bytenr>::
69 use the given offset 'bytenr' for the tree root
73 run in read-only mode, this option exists to calm potential panic when users
74 are going to run the checker
76 -s|--super <superblock>::
77 use 'superblock'th superblock copy, valid values are 0, 1 or 2 if the
78 respective superblock offset is within the device size
80 This can be used to use a different starting point if some of the primary
81 superblock is damaged.
83 --clear-space-cache v1|v2::
84 completely wipe all free space cache of given type
86 For free space cache 'v1', the 'clear_cache' kernel mount option only rebuilds
87 the free space cache for block groups that are modified while the filesystem is
88 mounted with that option. Thus, using this option with 'v1' makes it possible
89 to actually clear the entire free space cache.
91 For free space cache 'v2', the 'clear_cache' kernel mount option does destroy
92 the entire free space cache. This option with 'v2' provides an alternative
93 method of clearing the free space cache that doesn't require mounting the
101 enable the repair mode and attempt to fix problems where possible
103 create a new checksum tree and recalculate checksums in all files
105 NOTE: Do not blindly use this option to fix checksum mismatch problems.
108 build the extent tree from scratch
110 NOTE: Do not use unless you know what you're doing.
113 select mode of operation regarding memory and IO
115 The 'MODE' can be one of 'original' and 'lowmem'. The original mode is mostly
116 unoptimized regarding memory consumption and can lead to out-of-memory
117 conditions on large filesystems. The possible workaround is to export the block
118 device over network to a machine with enough memory. The low memory mode is
119 supposed to address the memory consumption, at the cost of increased IO when it
120 needs to re-read blocks when needed. This may increase run time.
122 NOTE: 'lowmem' mode does not work with '--repair' yet, and is still considered
126 allow to work on a mounted filesystem. Note that this should work fine on a
127 quiescent or read-only mounted filesystem but may crash if the device is
128 changed externally, eg. by the kernel module. Repair without mount checks is
129 not supported right now.
133 *btrfs check* returns a zero exit status if it succeeds. Non zero is
134 returned in case of failure.
138 *btrfs* is part of btrfs-progs.
139 Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for