On Sat, Feb 15, 2014 at 5:04 PM, Mikael Abrahamsson [off-list ref] wrote:

On Sat, 15 Feb 2014, Jon Nelson wrote:

quoted

Out of 12 drives, I thought RAID6 only offered a total of *2* failed
devices. It seems to me that you have 7 devices in sync and 4 *almost* in
sync. It's this "almost" part that has me confused. How can the raid run if
the event count doesn't match? Wouldn't at least 10 out of 12 drives have to
have the same event count to avoid data loss?

Correct. When you use --assemble --force you're basically telling mdadm "I
know what I'm doing and I'll take the risk of data loss or corruption". If
you assemble in with a kicked drive that was kicked long ago that has a
really far off event count, you can really really screw things up.

Unless you use --force, mdadm won't assemble an array where the event count
doesn't match up.

Aha. So if you ran a check in this case, it would find some number of
blocks that don't match up. What does MD do in that case?

-- 
Jon