Re: On URE and RAID rebuild - again!

[NeilBrown <neilb@suse.de>]

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On Wed, 30 Jul 2014 15:05:29 +0200 Gionatan Danti <g.danti@assyoma.it> wrote:

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

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