Re: Creating a 3-disk RAID6 array

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 05/17/2012 07:04 AM, John Robinson wrote:
> On 17/05/2012 14:18, David Brown wrote:
> [...]
>> Theoretically, a 3-disk RAID6 is like a 2-disk RAID5 or a 1-disk RAID1 -
> [...]
>> I can't think of any good reason for it /not/ to support 3-disk
>> RAID6, as there is nothing in the algorithms to hinder it.
> 
> I've a vague recollection of Peter Anvin saying the implementation is
> optimised in such a way that it won't work.
> 

Yes, it would have required introducing some odd special cases.  A
3-disk RAID-6 is bitwise identical to a 3-disk RAID-1, so if Neil wants
to he could add "instant reshaping" support in mdadm (add a disk to a
3-disk RAID-1 turning it into a 4-disk RAID-6; and similar for 2-disk
RAID-1 into 3-disk RAID-5.)

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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