btrfs-progs/Documentation/btrfs-select-super.asciidoc

btrfs-select-super(8)
=====================

NAME
----
btrfs-select-super - overwrite primary superblock with a backup copy

SYNOPSIS
--------
*btrfs-select-super* -s number <device>

DESCRIPTION
-----------
Destructively overwrite all copies of the superblock
with a specified copy.  This helps in certain cases, for example when write
barriers were disabled during a power failure and not all superblocks were
written, or if the primary superblock is damaged, eg. accidentally overwritten.

The filesystem specified by 'device' must not be mounted.

NOTE: *Prior to overwriting the primary superblock, please make sure that the backup
copies are valid!*

To dump a superblock use the *btrfs inspect-internal
dump-super* command, or the obsolete command *btrfs-show-super*.

Then run the check (in the non-repair mode) using the command *btrfs check -s*
where '-s' specifies the superblock copy to use.

Superblock copies exist in the following offsets on the device:

- primary: '64KiB' (65536)
- 1st copy: '64MiB' (67108864)
- 2nd copy: '256GiB' (274877906944)

A superblock size is '4KiB' (4096).

OPTIONS
-------
-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

SEE ALSO
--------
`btrfs-inspect-internal`(8),
`btrfsck check`(8)