Re: Two identical systems, radically different performance

On Oct 9, 2012, at 3:24 AM, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:

> On 9.10.2012 00:33, Evgeny Shishkin wrote:
>>>
>>> pgbench: Old server
>>>
>>> pgbench -i -s 100 -U test
>>> pgbench -U test -c ... -t ...
>>>
>>> -c -t TPS
>>> 5 20000 3777
>>> 10 10000 2622
>>> 20 5000 3759
>>> 30 3333 5712
>>> 40 2500 5953
>>> 50 2000 6141
>>>
>>> New server
>>> -c -t TPS
>>> 5 20000 2733
>>> 10 10000 2783
>>> 20 5000 3241
>>> 30 3333 2987
>>> 40 2500 2739
>>> 50 2000 2119
>>
>> On new server postgresql do not scale at all. Looks like contention.
>
> Why? The evidence we've seen so far IMHO suggests a poorly performing
> I/O subsystem. Post a few lines of "vmstat 1" / "iostat -x -k 1"
> collected when the pgbench is running, that might tell us more.
>

Because 50 clients can push io even with small read ahead. And hear we see nice parabola. Just guessing anyway.

> Try a few very basic I/O tests that are easy to understand rather than
> running bonnie++ which is quite complex. For example try this:
>
> time sh -c "dd if=/dev/zero of=myfile.tmp bs=8192 count=4194304 && sync"
>
> dd if=myfile.tmp of=/dev/null bs=8192
>
> The former measures sequential write speed, the latter measures
> sequential read speed in a very primitive way. Watch vmstat/iostat and
> don't bother running pgbench until you get a reasonable performance on
> both systems.
>
>
> Tomas
>
>
> --
> Sent via pgsql-performance mailing list (pgsql-performance(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-performance