Re: RAID-1 to RAID-5 online migration?
On Mon, 2004-09-13 at 00:41, Russell Coker wrote:
> On Mon, 6 Sep 2004 23:35, Adrian 'Dagurashibanipal' von Bidder
> <avbidder@fortytwo.ch> wrote:
[...]
> Do you have benchmark results to support this assertion? Last time I tested
> the performance of software RAID-1 on Linux I was unable to get anywhere near
> 2x disk speed for writing. I did tests by reading two files that were 1G in
> size and the operation took considerably longer than reading a single 1G file
> from a non-RAID system. If RAID-1 was delivering twice the read throughput
> then I should be able to read two 1G files concurrently from a RAID-1 in the
> same time as would be taken to read a single 1G file from a single disk.
I think Russel must be checking if the class is awake :-)
That doesn't sound like a fair test; reading two files at once means the
heads have to bounce around all over the place.
If you are just talking throughput, then reading a 1G file should take
half the time on a RAID-1 that it does on a single disk.
I suspect that reading 2 1G files at once on RAID-1 will be not much
faster than reading 2 1G files on a single disk, because reading two
files at once will probably be seek-bound, not throughput bound. RAID-1
boosts throughput, not latency.
HDD latency is a killer. It is significantly faster to read small
objects from another machine's RAM over ethernet than off the local HDD;
HDD latency is ~10ms, ethernet is <1ms.
--
Donovan Baarda <abo@minkirri.apana.org.au>
http://minkirri.apana.org.au/~abo/
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