btrfs-progs/Documentation/btrfs-rescue.asciidoc

btrfs-rescue(8)
==============

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.
-v::::
verbose mode.
-h::::
help.

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

*super-recover* [options] <device>::
Recover bad superblocks from good copies.
+
`Options`
+
-y::::
assume an answer of 'yes' to all questions.
-v::::
verbose mode.

*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 stacktraces that may show up in system log.
+
The common case where this happens has been fixed a long time ago,
so it is unlikely that you will see this particular problem, but the utility 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'.

*fix-device-size* <device>::
fix device size and super block total bytes
+
This command will fix the following problems, by re-aligning all devices' total
bytes and re-calculating super block total bytes.
+
1. Newer kernel refuse to mount btrfs caused by mismatch super block total bytes
+
----
BTRFS error (device sdb): super_total_bytes 92017859088384 mismatch with fs_devices total_rw_bytes 92017859094528
----
+
2. Noisy kernel warning for newer kernels
+
----
WARNING: CPU: 3 PID: 439 at fs/btrfs/ctree.h:1559 btrfs_update_device+0x1c5/0x1d0 [btrfs]
----
+
And the corresponding line is the `WARN_ON()` line below:
+
----
{
	BUILD_BUG_ON(sizeof(u64) !=
		     sizeof(((struct btrfs_dev_item *)0))->total_bytes);
	WARN_ON(!IS_ALIGNED(val, eb->fs_info->sectorsize));
	btrfs_set_64(eb, s, offsetof(struct btrfs_dev_item, total_bytes), val);
}
----

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 btrfs wiki http://btrfs.wiki.kernel.org for
further details.

SEE ALSO
--------
`mkfs.btrfs`(8),
`btrfs-scrub`(8),
`btrfs-check`(8)