RAID-6

Hi all,

I'm playing around with RAID-6 algorithms lately.  With RAID-6 I mean
a setup which needs N+2 disks for N disks worth of storage and can
handle any two disks failing -- this seems to be the contemporary
definition of RAID-6 (the originally proposed "two-dimensional parity"
which required N+2*sqrt(N) drives never took off for obvious reasons.)

Based on my current research, I think the following should be true:

a) write performance will be worse than RAID-5, but I believe it can
   be kept to within a factor of 1.5-2.0 on machines with suitable
   SIMD instruction sets (e.g. MMX or SSE-2);

b) read performance in normal and single failure degraded mode will be
   comparable to RAID-5;

c) read performance in dual failure degraded mode will be quite bad.

I'm curious how much interest there would be in this, since I
certainly have enough projects without it, and I'm probably going to
need some of Neil's time to integrate it into the md driver and the
tools.

	-hpa
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