Greg Smith wrote:
> On Mon, 5 May 2008, Craig James wrote:
>
>> pgbench -i -s 20 -U test
>
> That's way too low to expect you'll see a difference in I/O schedulers.
> A scale of 20 is giving you a 320MB database, you can fit the whole
> thing in RAM and almost all of it on your controller cache. What's
> there to schedule? You're just moving between buffers that are
> generally large enough to hold most of what they need.

Test repeated with:
autovacuum enabled
database destroyed and recreated between runs
pgbench -i -s 600 ...
pgbench -c 10 -t 50000 -n ...

I/O Sched AVG Test1 Test2
--------- ----- ----- -----
cfq 705 695 715
noop 758 769 747
deadline 741 705 775
anticipatory 494 477 511

I only did two runs of each, which took about 24 minutes. Like the first round of tests, the "noise" in the measurements (about 10%) exceeds the difference between scheduler-algorithm performance, except that "anticipatory" seems to be measurably slower.

So it still looks like cfq, noop and deadline are more or less equivalent when used with a battery-backed RAID.

Craig

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