btrfs-progs: docs: update btrfs-select-super

Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/Documentation/btrfs-select-super.asciidoc b/Documentation/btrfs-select-super.asciidoc
index a8d7ef0..6e94a03 100644 (file)
--- a/Documentation/btrfs-select-super.asciidoc
+++ b/Documentation/btrfs-select-super.asciidoc
@@ -3,27 +3,45 @@ btrfs-select-super(8)
 
 NAME
 ----
-btrfs-select-super - overwrite superblock with a backup
+btrfs-select-super - overwrite primary superblock with a backup copy
 
 SYNOPSIS
 --------
-*btrfs-select-super* -s number dev
+*btrfs-select-super* -s number <device>
 
 DESCRIPTION
 -----------
-*btrfs-select-super* destructively overwrites all copies of the superblock
-with a specified copy.  This helps with certain cases of damage, especially
-when barriers were disabled during a power failure.  You can find a valid
-copy of the superblock with *btrfs check -s*.
+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 `dev` must not be mounted.
+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 up to 2 if the
-respective superblock offset is within the filesystem
+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)