On Wed, 30 Jul 2014 15:05:29 +0200 Gionatan Danti [off-list ref] wrote:

On 30/07/2014 13:13, Mikael Abrahamsson wrote:

quoted

There has been much discussion about the URE figures. Some people
interpret it one way, others another way. There is nobody here that
knows for sure. Ask your HDD vendor, if they answer, do share here!

- Ouch! -
I was hoping that HDD vendors were somewhat more open about their URE 
calculations... It's time for some lab test, I think!
 >

quoted

When MD encounters an URE, it should calculate that block from parity
information and write it. I have personally had problems with this not
happening, seems it might be that if the URE doesn't happen repeatedly,
MD might not re-write. All parity raid levels should behave the same, so
this should work identically for RAID1, RAID10, RAID5 and RAID6.

What about _degraded_ array state? In other words, if a degraded RAID5 
experiences a URE during rebuild, what happens? I read that most 
hardware based RAID card both stop rebuilding _and_ kill the entire 
array. From my understanding, mdadm should stop rebuilding but the array 
can the restarted, mounted and backupped. Right?

Regards.

Yes, you can usually get your data back with mdadm.

With latest code, a URE during recovery will cause a bad-block to be recorded
on the recovered device, and recovery will continue.  You end up with a
working array that has a few unreadable blocks on it.

NeilBrown