Assemble: add support for RAID0 layouts.

If you have a RAID0 array with varying sized devices
on a kernel before 5.4, you cannot assembling it on
5.4 or later without explicitly setting the layout.
This is now possible with
  --update=layout-original (For 3.13 and earlier kernels)
or
  --update=layout-alternate (for 3.14 and later kernels)

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

1 files changed, 17 insertions, 0 deletions

diff --git a/mdadm.8.in b/mdadm.8.in
index fc9b6a66..6b63bb41 100644
--- a/mdadm.8.in
+++ b/mdadm.8.in
@@ -1213,6 +1213,8 @@ argument given to this flag can be one of
 .BR no\-bbl ,
 .BR ppl ,
 .BR no\-ppl ,
+.BR layout\-original ,
+.BR layout\-alternate ,
 .BR metadata ,
 or
 .BR super\-minor .
@@ -1364,6 +1366,21 @@ The
 .B no\-ppl
 option will disable PPL in the superblock.
 
+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,
+.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
+.B \-\-update=layout\-alternate
+must be given.  This only needs to be given once.  Subsequent assembly of the array
+will happen normally.
+For more information, see
+.IR md (4).
+
 .TP
 .BR \-\-freeze\-reshape
 Option is intended to be used in start-up scripts during initrd boot phase.