On Tue, 28 May 2013 13:50:49 +0300 Alexander Lyakas [off-list ref]
wrote:

Neil,
In my opinion (I may be wrong), a spare drive (raid_disk==-1) doesn't
add any information to array assembly. It doesn't have a valid raid
slot, and I don't see how its event count is relevant. I don't think a
spare can help us much in figuring out array's latest state, which is
what assembly code tries to do.
So what I was thinking: mdadm --assemble doesn't consider spare drives
(raid_disk=-1) at all. It simply skips over them in the initial loop
after reading their superblocks. Perhaps it can keep them in a side
list. Then array is assembled with non-spare drives only.

After array is assembled, we may choose one of the following:
# User has to explicitly add the spare drives after array has been
assembled. Assemble can warn that some spares have been left out, and
tell the user what they are.
# Assemble adds the spare drives (perhaps after zeroing their
superblocks even), after it assembled the array with non-spare drives.

Hi,
 I have just committed

  http://git.neil.brown.name/?p=mdadm.git;a=commitdiff;h=f80057aec5d314798251e318555cb8ac92e4c06f

which I believe fixes this issue.  If you can test and confirm I would
appreciate it.

NeilBrown