Re: Higher than expected disk write(2) latency
From: Martin Sustrik
Date: Mon Jun 30 2008 - 14:11:18 EST
Hi Roger,
If these figures are to be believed, then why are we seeing latencies of
8.3 msec? Is this normal? Or are we just being overly optimistic in
our performance expectations?
Consider this, 60/7200rpm=8.3ms for one rotation.
You write sector n and n+1, it takes some amount of time for that first
set of sectors to come under the head, when it does you write it and
immediately return. Immediately after that you attempt write sector
n+2 and n+3 which just a bit ago passed under the head, so you have to
wait an *ENTIRE* revolution for those sectors to again come under the
head to be written, another ~8.3ms, and you continue to repeat this with
each block being written. If the sector was randomly placed in the
rotation (ie 50% chance of the disk being off by 1/2 a rotation or
less-you would have a 4.15 ms average seek time for your test)-but the
case of sequential sync writes this leaves the sector about as far as
possible from the head (it just passed under the head).
Fair enough. That exaplains the behaviour. Would AIO help here? If we
are able to enqueue next write before the first one is finished, it can
start writing it immediately without waiting for a revolution.
We also ran the same test on a different system with recent SAS disks
connected via a HP/Compaq CCISS controller. I don't have the exact
details of the drives used, since I don't know how to get them out of
the cciss driver, but the latencies we got were around 4 msec. Whilst
this is better than the "commodity" hardware used in the tests above, it
still seems excessive.
Almost the same case as for the 7200 rpm disk, but I bet these SAS
drives are 15k drives? If so 60/15000=4ms.
Bingo!
Thanks.
Martin
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