Assemble: allow an array undergoing reshape to be started without backup file

Though not having the proper backup file can cause data corruption, it
is not enough to justify not being able to start the array at all.
So allow "--invalid-backup" to be specified which says "just continue
even if a backup cannot be restored".

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 13 insertions, 2 deletions

diff --git a/mdadm.8.in b/mdadm.8.in
index 4fc94396..8a75e256 100644
--- a/mdadm.8.in
+++ b/mdadm.8.in
@@ -884,14 +884,25 @@ bitmap, there is no need to specify this when assembling the array.
 .BR \-\-backup\-file=
 If
 .B \-\-backup\-file
-was used to grow the number of raid-devices in a RAID5, and the system
-crashed during the critical section, then the same
+was used while reshaping an array (e.g. changing number of devices or
+chunk size) and the system crashed during the critical section, then the same
 .B \-\-backup\-file
 must be presented to
 .B \-\-assemble
 to allow possibly corrupted data to be restored.
 
 .TP
+.BR \-\-invalid\-backup
+If the file needed for the above option is not available for any
+reason an empty file can be given together with this option to
+indicate that the backup file is invalid.  In this case the data that
+was being rearranged at the time of the crash could be irrecoverably
+lost, but the rest of the array may still be recoverable.  This option
+should only be used as a last resort if there is no way to recover the
+backup file.
+
+
+.TP
 .BR \-U ", " \-\-update=
 Update the superblock on each device while assembling the array.  The
 argument given to this flag can be one of