Re: strange performance regression between 7.4 and 8.1

On Thu, 1 Mar 2007, Joshua D. Drake wrote:

> Alex Deucher wrote:
>> Hello,
>>
>> I have noticed a strange performance regression and I'm at a loss as
>> to what's happening. We have a fairly large database (~16 GB). The
>> original postgres 7.4 was running on a sun v880 with 4 CPUs and 8 GB
>> of ram running Solaris on local scsi discs. The new server is a sun
>> Opteron box with 4 cores, 8 GB of ram running postgres 8.1.4 on Linux
>> (AMD64) on a 4 Gbps FC SAN volume. When we created the new database
>> it was created from scratch rather than copying over the old one,
>> however the table structure is almost identical (UTF8 on the new one
>> vs. C on the old). The problem is queries are ~10x slower on the new
>> hardware. I read several places that the SAN might be to blame, but
>> testing with bonnie and dd indicates that the SAN is actually almost
>> twice as fast as the scsi discs in the old sun server. I've tried
>> adjusting just about every option in the postgres config file, but
>> performance remains the same. Any ideas?
>
> Vacuum? Analayze? default_statistics_target? How many shared_buffers?
> effective_cache_size? work_mem?

Also, an explain analyze from both the 7.4 and 8.1 systems with one of the
10x slower queries would probably be handy.

What do you mean by "created from scratch rather than copying over the old
one"? How did you put the data in? Did you run analyze after loading it?
Is autovacuum enabled and if so, what are the thresholds?

--
Jeff Frost, Owner <jeff(at)frostconsultingllc(dot)com>
Frost Consulting, LLC http://www.frostconsultingllc.com/
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