Re: RAID6 questions

[Goswin von Brederlow <goswin-v-b@web.de>]

Luca Berra <bluca@comedia.it> writes:

> On Thu, Jul 02, 2009 at 05:22:54PM +0200, Marek wrote:
>>5. In case one decides for a partitioned approach - does mdadm kick
>>out faulty partitions or whole drives? I have read several sources
>>including some comments on slashdot that it's much better to split
>>large drives into many small partitions, but noone clarified in
> maybe because those suggesting this are not able to come up with a
> reasonable explanation?
>>detail.  A possible though unlikely scenario would be simultaneous
>>failure of all hdds in the array:
>>
>> md1 RAID6 sda1[_] sdb1[_] sdc1[U] sdd1[U] sde1[U] sdf1[U]
>> md2 RAID6 sda2[U] sdb2[_] sdc2[_] sdd2[U] sde2[U] sdf2[U]
>> md3 RAID6 sda3[U] sdb3[U] sdc3[_] sdd3[_] sde3[U] sdf3[U]
>> md4 RAID6 sda4[U] sdb4[U] sdc4[U] sdd4[_] sde4[_] sdf4[U]
>> md5 RAID6 sda5[U] sdb5[U] sdc5[U] sdd5[U] sde5[_] sdf5[_]
>>(...)
>>
>>If mdadm kicks out faulty partitions only, but leaves the remaining
>>part of drive going as long as it's able to read it, would it mean
>>that even if every single hdd in the array failed somewhere (for
>>example due to Reallocated_Sector_Ct), mdadm would keep the healthy
>>partitions of that failed drive running, thus the entire system would
>>be still running in degraded mode without loss of data?
> This really depends on your priorities, i would have replaced my drives
> well in advance of a similar situation.
> The only reason i can imagine for splitting a disk into many partitions
> and raiding them together is avoiding lenghty rebuilds when a single
> drive is kicked from an array due to a correctable read error.
> In practice the above scenario should not happen anymore, since md will
> retry writing a stripe if it gets a read-error, besides you are planning
> on using raid6, so a single drive failure will still leave you with a
> nice degree of protection.
>
> Regards,
> L.

Plus bitmaps do that much better.

MfG
        Goswin