Re: Triple parity and beyond

[David Brown <david.brown@hesbynett.no>]

On 20/11/13 02:23, John Williams wrote:
> On Tue, Nov 19, 2013 at 4:54 PM, Chris Murphy <lists@colorremedies.com>
> wrote:
>> If anything, I'd like to see two implementations of RAID 6 dual
>> parity. The existing implementation in the md driver and btrfs could
>> remain the default, but users could opt into Cauchy matrix based dual
>> parity which would then enable them an easy (and live) migration path
>> to triple parity and beyond.

Andrea's Cauchy matrix is compatible with the existing Raid6, so there
is no problem there.

I believe it would be a terrible idea to have an incompatible extension
- that would mean you could not have temporary extra parity drives with
asymmetrical layouts, which is something I see as a very useful feature.

> 
> Actually, my understanding is that Andrea's Cauchy matrix technique
> (call it C) is compatible with existing md RAID5 and RAID6 (call these
> A). It is only the non-SSSE3 triple-parity algorithm 2^-1 (call it B)
> that is incompatible with his Cauchy matrix technique.
> 
> So, you can have:
> 
> 1) A+B
> 
> or
> 
> 2) A+C
> 
> But you cannot have A+B+C

Yes, that's right.