Re: Seeking performance advice and explanation for high I/O on 8.3

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Scott Otis wrote:
Sorry about not responding to the whole list earlier - this is my first time posting to a mailing list.

Would providing more information about the size and complexities of the databases help?

I measure I/O stats with iostat - here is the command I use:

iostat -d -x mfid0 -t 290 2

I tried looking at the man page for iostat but couldn't find anywhere how to determine what the stats are for sequential vs random - any help there?

When using 'top -m io' the postgres stats collector process is constantly at 99% - 100%.

When using 'top' the WCPU for the postgres stats collector and the autovacuum process are constantly at 20% - 21%.

Is that normal?  It seems to me that the stats collector is doing all the I/O (which would mean the stats collector is doing 46.1 megabytes /sec).

Also, the I/O stats don't change hardly at all (except at night during backups which makes sense).  They don't go up or down with user activity on the server - which makes me wonder a little bit.  I have a feeling that if I just turned off Apache that the I/O stats wouldn't change.  Which leads me to believe that the I/O is not query related - its stats collecting and autovacuuming related.  Is that expected?

It seems to me that the stats collector shouldn't be using that much I/O and CPU (and the autovacuum shouldn't be using that much CPU)  - therefore something in my configuration must be messed up or could be changed somehow.  But maybe I'm wrong - please let me know.

I don't think my setup is necessarily slow.  I just want to make it as efficient as possible and wanted to get some feedback to see if am setting things up right.  I am also looking out into the future and seeing how much load I can put on this server before getting another one.  If I can reduce the I/O and CPU that the stats collector and autovacuum are using without losing any functionality then I can put more load on the server.

Again thanks for all the help.

Scott Otis
CIO / Lead Developer
Intand
www.intand.com


When using 'top -m io' the postgres stats collector process is constantly at 99% - 100%.
When using 'top' the WCPU for the postgres stats collector and the autovacuum process are constantly at 20% - 21%.

Yeah, that sounds excessive.  But my database gets 20 transactions a DAY, so, I have no experience with a busy box.

You say its mostly selects, but do you have any triggers or anything that might update a table?  Do you do inserts or updates to track traffic?

What does:

select * from pg_stat_activity

look like? (I think vacuum will show up in there, right?)  I'm curious if we can find the table autovacuum is working on, maybe that'll help pin it down.

-Andy

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