Manpage/btrfs-check

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<p><b>btrfsck</b> is an alias of <b>btrfs check</b> command and is now deprecated.</p>
 
<p><b>btrfsck</b> is an alias of <b>btrfs check</b> command and is now deprecated.</p>
 
<blockquote><b>Warning:</b>
 
<blockquote><b>Warning:</b>
Do not use <em>--repair</em> unless you are advised to by a developer, an
+
Do not use <em>--repair</em> unless you are advised to do so by a developer
experienced user or accept the fact that <em>fsck</em> cannot possibly fix all sorts
+
or an experienced user, and then only after having accepted that no <em>fsck</em>
of damage that could happen to a filesystem because of software and hardware
+
successfully repair all types of filesystem corruption. Eg. some other software
bugs.</blockquote>
+
or hardware bugs can fatally damage a volume.</blockquote>
 
<p>The structural integrity check verifies if internal filesystem objects or
 
<p>The structural integrity check verifies if internal filesystem objects or
 
data structures satisfy the constraints, point to the right objects or are
 
data structures satisfy the constraints, point to the right objects or are
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connectivity etc.</p>
 
connectivity etc.</p>
 
<p>The amount of memory required can be high, depending on the size of the
 
<p>The amount of memory required can be high, depending on the size of the
filesystem, similarly the run time.</p>
+
filesystem, similarly the run time. Check the modes that can also affect that.</p>
 
==SAFE OR ADVISORY OPTIONS==
 
==SAFE OR ADVISORY OPTIONS==
  
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verify checksums of data blocks
 
verify checksums of data blocks
 
</p>
 
</p>
<p>This expects that the filesystem is otherwise
+
<p>This expects that the filesystem is otherwise OK, and is basically and offline
OK, so this is basically and offline <em>scrub</em> but does not repair data from
+
<em>scrub</em> but does not repair data from spare copies.</p>
spare copies.</p>
+
  
 
<dt>
 
<dt>
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mounted with that option. Thus, using this option with <em>v1</em> makes it possible
 
mounted with that option. Thus, using this option with <em>v1</em> makes it possible
 
to actually clear the entire free space cache.</p>
 
to actually clear the entire free space cache.</p>
<p>For free space cache <em>v2</em>, the <em>clear_cache</em> kernel mount option does destroy
+
<p>For free space cache <em>v2</em>, the <em>clear_cache</em> kernel mount option destroys
the entire free space cache. This option with <em>v2</em> provides an alternative
+
the entire free space cache. This option, with <em>v2</em> provides an alternative
 
method of clearing the free space cache that doesn&#8217;t require mounting the
 
method of clearing the free space cache that doesn&#8217;t require mounting the
 
filesystem.</p>
 
filesystem.</p>
Line 131: Line 130:
 
enable the repair mode and attempt to fix problems where possible
 
enable the repair mode and attempt to fix problems where possible
 
</p>
 
</p>
 +
<blockquote><b>Note:</b>
 +
there&#8217;s a warning and 10 second delay when this option is run without
 +
<em>--force</em> to give users a chance to think twice before running repair, the
 +
warnings in documentation have shown to be insufficient</blockquote>
  
 
<dt>
 
<dt>
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<dt>
 
<dt>
--mode=MODE
+
--mode <em>&lt;MODE&gt;</em>
 
<dd>
 
<dd>
 
<p>
 
<p>
 
select mode of operation regarding memory and IO
 
select mode of operation regarding memory and IO
 
</p>
 
</p>
<p>The <em>MODE</em> can be one of <em>original</em> and <em>lowmem</em>. The original mode is mostly
+
<p>The <em>MODE</em> can be one of:</p>
unoptimized regarding memory consumption and can lead to out-of-memory
+
<dl>
conditions on large filesystems. The possible workaround is to export the block
+
<dt>
device over network to a machine with enough memory. The low memory mode is
+
<em>original</em>
supposed to address the memory consumption, at the cost of increased IO when it
+
<dd>
needs to re-read blocks when needed. This may increase run time.</p>
+
<p>
 +
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 memory.
 +
</p>
  
</dl>
+
<dt>
 +
<em>lowmem</em>
 +
<dd>
 +
<p>
 +
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.
 +
</p>
 
<blockquote><b>Note:</b>
 
<blockquote><b>Note:</b>
 
<em>lowmem</em> mode does not work with <em>--repair</em> yet, and is still considered
 
<em>lowmem</em> mode does not work with <em>--repair</em> yet, and is still considered
 
experimental.</blockquote>
 
experimental.</blockquote>
<dl>
+
 
 +
</dl>
 +
 
 
<dt>
 
<dt>
 
--force
 
--force
 
<dd>
 
<dd>
 
<p>
 
<p>
allow to work on a mounted filesystem. Note that this should work fine on a
+
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
 
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
 
changed externally, eg. by the kernel module.  Repair without mount checks is
 
not supported right now.
 
not supported right now.
 
</p>
 
</p>
 +
<p>This option also skips the delay and warning in the repair mode (see
 +
<em>--repair</em>).</p>
  
 
</dl>
 
</dl>

Latest revision as of 17:38, 16 January 2020

Contents

[edit] btrfs-check(8) manual page

Warning: this page is automatically generated from git, all edits will be lost. Current git version.


[edit] NAME

btrfs-check - check or repair a btrfs filesystem

[edit] SYNOPSIS

btrfs check [options] <device>

[edit] DESCRIPTION

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 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 affect that.

[edit] 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, and 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 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.

[edit] DANGEROUS OPTIONS

--repair

enable the repair mode and attempt to fix problems where possible

Note: 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 be insufficient

--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

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 memory.

lowmem

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.

Note: lowmem mode does not work with --repair yet, and is still considered experimental.

--force

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 (see --repair).

[edit] EXIT STATUS

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

[edit] AVAILABILITY

btrfs is part of btrfs-progs. Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for further details.

[edit] SEE ALSO

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

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