Re: Database size Vs performance degradation

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Matthew
Wakeling
Sent: July 30, 2008 8:37 AM
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] Database size Vs performance degradation

On Wed, 30 Jul 2008, Dave North wrote:
> Running on HP DL380 w/ 4GB RAM, dual 10K HDDs in RAID 0+1

> Checking the stats, the DB size is around 7.5GB;

Doesn't fit in RAM.

> ...after the load, the DB size was around 2.7GB

Does fit in RAM.

> One observation I've made on the DB system is the disk I/O seems
> dreadfully slow...we're at around 75% I/O wait sometimes and the read
> rates seem quite slow (hdparm says around 2.2MB/sec - 20MB/sec for
> un-cached reads).

That's incredibly slow in this day and age, especially from 10krpm HDDs.

Definitely worth investigating.

DN: Yeah, I was thinking the same thing. Unlike the folks here, I'm no
performance whiz but it did seem crazy slow. Given the 10K disks, it
seems to me there is most likely something on the RAID Array itself that
is set sub-optimally. Next thing to look at.

However, I think vacuuming more agressively is going to be your best win
at the moment.

DN: As I just replied to the past (very helpful) chap, I think I need to
go see what exactly the vac is vac'ing (autovac that is) because
although it's running super frequently, the big question is "is it doing
anything" :)

Cheers

Dave