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

SYNOPSIS
--------

**btrfs rescue** <subcommand> <args>

DESCRIPTION
-----------

A set of commands that are targeting to fix a specific problem and may not
suitable for :doc`btrfs-check`.

.. note::
   All the subcommands cannot handle an ongoing replace/balance, please finish/cancel
   the replace/balance before using this command.

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 :command:`chunk-recover` will scan the whole device, it will be very
   slow especially if 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.

        .. code-block:: none

                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:

        .. code-block:: none

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

fix-data-checksum <device>
	Selectively fix data checksum mismatch.

	There is a long existing problem that if a user space program is doing
	direct IO and modifies the buffer before the write back finished, it
	can lead to data checksum mismatches.

	This problem is known but not fixed until upstream release v6.15
	(backported to older kernels). So it's possible to hit false data
	checksum mismatch for any long running btrfs.

	In that case this program can be utilized to repair such problem.

        ``Options``

	-r|--readonly
		readonly mode, only scan for and report data checksum mismatches,
		do not repair

	-i|--interactive
		interactive mode, ask for how to repair, ignore the errors by default

	-m|--mirror <num>
		use specified mirror to update the checksum item for all corrupted blocks.

		The value must be >= 1, and if the corrupted block has fewer mirrors than
		the value, the mirror number will be ``num % (num_mirrors + 1)``.

.. _man-rescue-clear-ino-cache:

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

        The 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.

.. _man-rescue-clear-uuid-tree:

clear-uuid-tree <device>
        Clear the UUID tree, so that kernel can regenerate 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.
        See also :docref:`mount option rescan_uuid_tree <btrfs-man5:mount-option-rescan-uuid-tree>`.

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 during log replay. See below
        for sample stack traces that may show up in system log.

        .. 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 :command:`zero-log` is needed according to the kernel
        backtrace:

        .. code-block:: none

                ? 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 :command:`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 <https://btrfs.readthedocs.io>`_.

SEE ALSO
--------

:doc:`btrfs-check`,
:doc:`btrfs-scrub`,
:doc:`mkfs.btrfs`
