Super1: allow RAID0 layout setting to be removed.

Once the RAID0 layout has been set, the RAID0 array cannot be assembled
on an older kernel which doesn't understand layouts.
This is an intentional safety feature, but sometimes people need the
ability to roll-back to a previously working configuration.

So add "--update=layout-unspecified" to remove RAID0 layout information
from the superblock.
Running "--assemble --update=layout-unspecified" will cause the assembly
the fail when run on a newer kernel, but will allow it to work on
an older kernel.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/mdadm.8.in b/mdadm.8.in
index ab832e85..34a93a8f 100644
--- a/mdadm.8.in
+++ b/mdadm.8.in
@@ -1213,6 +1213,7 @@ argument given to this flag can be one of
 .BR no\-ppl ,
 .BR layout\-original ,
 .BR layout\-alternate ,
+.BR layout\-unspecified ,
 .BR metadata ,
 or
 .BR super\-minor .
@@ -1368,8 +1369,9 @@ The
 .B layout\-original
 and
 .B layout\-alternate
-options are for RAID0 arrays in use before Linux 5.4.  If the array was being
-used with Linux 3.13 or earlier, then to assemble the array on a new kernel,
+options are for RAID0 arrays with non-uniform devices size that were in
+use before Linux 5.4.  If the array was being used with Linux 3.13 or
+earlier, then to assemble the array on a new kernel, 
 .B \-\-update=layout\-original
 must be given.  If the array was created and used with a kernel from Linux 3.14 to
 Linux 5.3, then
@@ -1379,6 +1381,15 @@ will happen normally.
 For more information, see
 .IR md (4).
 
+The
+.B layout\-unspecified
+option reverts the effect of
+.B layout\-orignal
+or
+.B layout\-alternate
+and allows the array to be again used on a kernel prior to Linux 5.3.
+This option should be used with great caution.
+
 .TP
 .BR \-\-freeze\-reshape
 Option is intended to be used in start-up scripts during initrd boot phase.