'\" t
.\"     Title: btrfs-select-super
.\"    Author: [FIXME: author] [see http://www.docbook.org/tdg5/en/html/author]
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 01/23/2019
.\"    Manual: Btrfs Manual
.\"    Source: Btrfs v4.20.1
.\"  Language: English
.\"
.TH "BTRFS\-SELECT\-SUPER" "8" "01/23/2019" "Btrfs v4\&.20\&.1" "Btrfs Manual"
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.nh
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.SH "NAME"
btrfs-select-super \- overwrite primary superblock with a backup copy
.SH "SYNOPSIS"
.sp
\fBbtrfs\-select\-super\fR \-s number \fI<device>\fR
.SH "DESCRIPTION"
.sp
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\&.
.sp
The filesystem specified by \fIdevice\fR must not be mounted\&.
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBNote\fR
.ps -1
.br
.sp
\fBPrior to overwriting the primary superblock, please make sure that the backup copies are valid!\fR
.sp .5v
.RE
.sp
To dump a superblock use the \fBbtrfs inspect\-internal dump\-super\fR command\&.
.sp
Then run the check (in the non\-repair mode) using the command \fBbtrfs check \-s\fR where \fI\-s\fR specifies the superblock copy to use\&.
.sp
Superblock copies exist in the following offsets on the device:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
primary:
\fI64KiB\fR
(65536)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
1st copy:
\fI64MiB\fR
(67108864)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
2nd copy:
\fI256GiB\fR
(274877906944)
.RE
.sp
A superblock size is \fI4KiB\fR (4096)\&.
.SH "OPTIONS"
.PP
\-s|\-\-super \fI<superblock>\fR
.RS 4
use \*(Aqsuperblock\(cqth superblock copy, valid values are 0 1 or 2 if the respective superblock offset is within the device size
.RE
.SH "SEE ALSO"
.sp
\fBbtrfs\-inspect\-internal\fR(8), \fBbtrfsck check\fR(8)