Re: proactive disk replacement

[Gandalf Corvotempesta <gandalf.corvotempesta@gmail.com>]

2017-03-21 14:02 GMT+01:00 David Brown <david.brown@hesbynett.no>:
> Note that to cause failure in non-degraded RAID5 (or degraded RAID6),
> your two URE's need to be on the same stripe in order to cause data
> loss.  The chances of getting an URE somewhere on the disk are roughly
> proportional to the size of the disk - but the chance of getting an URE
> on the same stripe as another URE on another disk are basically
> independent of the disk size, and it is extraordinarily small.

Little bit OT:
is this the same even for HW RAID Controllers like LSI Megaraid
or they tend to fail the rebuild in case of multiple URE even in
different stripes?

> No, you cannot.  Your conclusion here is based on several totally
> incorrect assumptions:
>
> 1. You think that RAID5/RAID6 recovery is more stressful, because the
> parity is "all over the place".  This is wrong.
>
> 2. You think that random IO has higher chance of getting an URE than
> linear IO.  This is wrong.

Totally agree.

> 3. You think that getting an URE on one disk, then getting an URE on a
> second disk, counts as a double failure that will break an single-parity
> redundancy (RAID5, RAID1, RAID6 in degraded mode).  This is wrong - it
> is only a problem if the two UREs are in the same stripe, which is quite
> literally a one in a million chance.

I'm not sure about this.
The posted paper is talking about "standard" raid made with hw raid controllers
and I'm not sure if they are able to finish a rebuild in case of double URE even
if coming from different stripes.

I think they fail the whole rebuild.