Re: 5 drives lost in an inactive 15 drive raid 6 system due to cable problem - how to recover?

On Wed, 08 Sep 2010 13:22:30 -0400
Norman White [off-list ref] wrote:

We have a 15 drive addonics array with 3 5 port sata multiplexors, one 
of the sas cables was knocked out to one of the port multiplexors and now
mdadm sees 9 drives , a spare, and 5 failed, removed drives (after 
fixing the cabling problem).

A mdadm -E on each of the drives, see 5 drives (the ones that were 
uncabled) as seeing the original  configuration with 14 drives and a 
spare, while the other 10 drives report
9 drives, a spare and 5 failed , removed drives.

We are very confident that there was no io going on at the time, but are 
not sure how to proceed.

One obvious thing to do is to just do a:

mdadm --assemble --force --assume-clean /dev/md0 sd[b,c, ... , p]
but we are getting different advice about what force will do in this 
situation. The last thing we want to do is wipe the array.

What sort of different advice?  From whom?

This should either do exactly what you want, or nothing at all.  I suspect
the former.  To be more confident I would need to see the output of
   mdadm -E /dev/sd[b-p]

NeilBrown

Another option would be to fiddle with the super blocks with mddump, so 
that they all see the same 15 drives in the same configuration, and then 
assemble it.

Yet another suggestion was to recreate the array configuration and hope 
that the data wouldn't be touched.

And even another suggestion is to create the array with one drive 
missing (so it is degraded and won't rebuild)

Any pointers on how to proceed would be helpful. Restoring 30TB takes 
along time.

Best,
Norman White
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