Re: Triple parity and beyond

On 20/11/13 02:23, John Williams wrote:

On Tue, Nov 19, 2013 at 4:54 PM, Chris Murphy [off-list ref]
wrote:

quoted

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.