Ubuntu Manpage: btrfs-rescue - recover a damaged btrfs filesystem

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Provided by: btrfs-progs_6.6.3-1.2_amd64

NAME

       btrfs-rescue - recover a damaged btrfs filesystem

SYNOPSIS

       btrfs rescue <subcommand> <args>

DESCRIPTION

       btrfs rescue is used to try to recover a damaged btrfs filesystem.

SUBCOMMAND

chunk-recover [options] <device>
Recover the chunk tree by scanning the devices

Options

-y assume an answer of yes to all questions.

-h help.

-v (deprecated) alias for global -v option

NOTE:
Since chunk-recover will scan the whole device, it will be very slow especially executed on a large
device.

fix-device-size <device>
fix device size and super block total bytes values that do not match

Kernel 4.11 starts to check the device size more strictly and this might mismatch the stored value
of total bytes. See the exact error message below. Newer kernel will refuse to mount the
filesystem where the values do not match. This error is not fatal and can be fixed. This command
will fix the device size values if possible.

BTRFS error (device sdb): super_total_bytes 92017859088384 mismatch with fs_devices total_rw_bytes 92017859094528

The mismatch may also exhibit as a kernel warning:

WARNING: CPU: 3 PID: 439 at fs/btrfs/ctree.h:1559 btrfs_update_device+0x1c5/0x1d0 [btrfs]

clear-ino-cache <device>
Remove leftover items pertaining to the deprecated inode cache feature.

The inode cache feature (enabled by mount option "inode_cache") has been completely removed in
5.11 kernel.

clear-space-cache <v1|v2> <device>
Completely remove the on-disk data of free space cache of given version.

Especially for v1 free space cache, clear_cache mount option would only remove the cache for
updated block groups, the remaining would not be removed. Thus this command is provided to
manually clear the free space cache.

clear-uuid-tree <device>
Clear UUID tree, so that kernel can re-generate it at next read-write mount.

Since kernel v4.16 there are more sanity check performed, and sometimes non-critical trees like
UUID tree can cause problems and reject the mount. In such case, clearing UUID tree may make the
filesystem to be mountable again without much risk as it's built from other trees.

super-recover [options] <device>
Recover bad superblocks from good copies.

Options

-y assume an answer of yes to all questions.

-v (deprecated) alias for global -v option

zero-log <device>
clear the filesystem log tree

This command will clear the filesystem log tree. This may fix a specific set of problem when the
filesystem mount fails due to the log replay. See below for sample stack traces that may show up
in system log.

The common case where this happens was fixed a long time ago, so it is unlikely that you will see
this particular problem, but the command is kept around.

NOTE:
Clearing the log may lead to loss of changes that were made since the last transaction commit.
This may be up to 30 seconds (default commit period) or less if the commit was implied by other
filesystem activity.

One can determine whether zero-log is needed according to the kernel backtrace:

? replay_one_dir_item+0xb5/0xb5 [btrfs]
? walk_log_tree+0x9c/0x19d [btrfs]
? btrfs_read_fs_root_no_radix+0x169/0x1a1 [btrfs]
? btrfs_recover_log_trees+0x195/0x29c [btrfs]
? replay_one_dir_item+0xb5/0xb5 [btrfs]
? btree_read_extent_buffer_pages+0x76/0xbc [btrfs]
? open_ctree+0xff6/0x132c [btrfs]

If the errors are like above, then zero-log should be used to clear the log and the filesystem may
be mounted normally again. The keywords to look for are 'open_ctree' which says that it's during
mount and function names that contain replay, recover or log_tree.

EXIT STATUS

       btrfs rescue 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 documentation at https://btrfs.readthedocs.io.