Re: emergency call for help: raid5 fallen apart

[Michael Evans <mjevans1983@gmail.com>]

On Thu, Feb 25, 2010 at 10:31 AM, John Robinson
<john.robinson@anonymous.org.uk> wrote:
> On 25/02/2010 17:41, Dawning Sky wrote:
>>
>> On Thu, Feb 25, 2010 at 8:45 AM, John Robinson
>> <john.robinson@anonymous.org.uk> wrote:
>>>
>>> On 25/02/2010 08:05, Giovanni Tessore wrote:
>>> [...]
>>> I do think we urgently need the hot reconstruction/recovery feature, so
>>> failing drives can be recovered to fresh drives with two sources of data,
>>> i.e. both the failing drive and the remaining drives in the array, giving
>>> us
>>> two chances of recovering every sector.
>>
>> I was one of those 4 cases in the part month.  I would have certainly
>> benefited from this when I tried to replace a failing drive on my old
>> raid-5.  But  I think actually the redundancy you desired can be
>> achieved by running a raid-6 at the degraded mode (with 1 missing
>> drive).
>>
>> Do I miss something?  If this is the case, shouldn't we all
>> be doing this instead of using the raid-5?
>
> I think you must be missing something, yes. RAID-6 with one drive missing
> would have 2 chances of recovering each sector, but then so does RAID-5 with
> no drives missing. In either case, lose a drive and you need every sector on
> the remaining drives to be good to complete the reconstruction and keep the
> array up.
>
> Cheers,
>
> John.
>
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No, what they're saying is that often drives don't /totally/ fail.
They have segments that go bad first, and we are often catching them
in that state.  To use the segments that /are/ successfully returned
there is a good chance that multiple 'not full member' drive could
provide a complete, or usefully very near complete with known 'dead'
areas set to store on fresh devices.
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