Mike,
you're right until you're using a single disk :)
Now, imagine you have more disks - more I/O operations you may perform, and
you'll need also a CPU time to process them :) until you fully use one CPU
per 'vacuumdb' - and then you stop...
As well, even in case when CPU is not highly used by vacuumdb - single process
is still not able to get a max performance of the storage array, just because
you need several concurrent I/O running in the system to reach max
throughput. And even filesystem might help you here - it's not all... More
concurrent writers you have - higher performance you reach (until real
limit)...
In my case I have a small storage array capable to give you more than
500MB/sec and say 5000 op/s. All my data are striped throw all array disks.
Single 'vacuumdb' process here become more CPU-bound rather I/O as it cannot
fully load storage array... So, more vacuum processes I start in parallel -
faster I'll finish database vacuuming.
Best regards!
-Dimitri
On Thursday 22 March 2007 18:10, Michael Stone wrote:
> On Thu, Mar 22, 2007 at 04:55:02PM +0100, Dimitri wrote:
> >In my case I have several CPU on the server and quite powerful storage box
> >which is not really busy with a single vacuum. So, my idea is quite simple
> > - speed-up vacuum with parallel execution (just an algorithm):
>
> Vacuum is I/O intensive, not CPU intensive. Running more of them will
> probably make things slower rather than faster, unless each thing you're
> vacuuming has its own (separate) disks. The fact that your CPU isn't
> pegged while vacuuming suggests that your disk is already your
> bottleneck--and doing multiple sequential scans on the same disk will
> definitely be slower than doing one.
>
> Mike Stone
>
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