Re: RAID-6

On Mon, Nov 11, 2002 at 10:52:36AM -0800, H. Peter Anvin wrote:

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);

Please note that raw CPU power is usually *not* a limiting (or even
significantly contributing) factor, on modern systems.

Limitations are disk reads/writes/seeks, bus bandwidth, etc.

You will probably cause more bus activity with RAID-6, and that might
degrade performance. But I don't think you need to worry about
MMX/SSE/...  If you can do as well as the current RAID-5 code, then you
will be in the clear until people have 1GB/sec disk transfer-rates on
500MHz PIII systems  ;)

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

Which again is like a RAID-0 with some extra seeks... Eg. not too bad
with huge chunk sizes.

You might want to consider using huge chunk-sizes when reading, but
making sure that writes can be made on "sub-chunks" - so that one could
run a RAID-6 with a 128k chunk size, yet have writes performed on 4k
chunks.  This is important for performance on both read and write, but
it is an optimization the current RAID-5 code lacks.

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.

I've seen quite some people ask for it.  You might find a friend in "Roy
Sigurd Karlsbach" - he for one has been asking (loudly) for it  ;)

Go Peter!    ;)

-- 
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