On 13/09/2012 09:43, John Robinson wrote:
On 13/09/2012 08:30, David Brown wrote:
[...]
Using 2 x 8-disk raid6 connected by
raid0 (or linear concat for XFS) will be faster and safer than a single
large raid5/6 array, but unless you use the latest multithreaded raid
kernel then it will still be very slow.
Not as far as I understand it, it won't. The multithreaded code is only
really a benefit on SSDs which can manage tens of thousands of IOPS,
while on spinning rust HDDs which can only manage hundreds of IOPS, the
single-threaded code is fine.
HDDs have longer latencies, so that can't do lots of different accesses
in rapid succession. But they have pretty high throughputs for streamed
reads and writes, once they get going. Raid6 writes involve quite a bit
of processing to calculate the second parity - when you have enough HD
spindles you will saturate the performance of a single CPU core in
processing speed, memory bandwidth and IO bandwidth. I freely admit
that I'm speculating here without real numbers, but I believe this cpu
bottleneck is part of the problems people see with larger raid5/6 arrays.
It is certainly true that you will see the biggest difference with SSDs,
especially in the IOPS numbers.
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