From: Janning <ml@planwerk6.de>
To: pgsql-performance@postgresql.org
Subject: Re: Write performance
Date: Thu, 24 Jun 2010 15:16:05 +0200
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On Thursday 24 June 2010 14:53:57 Matthew Wakeling wrote:
> On Thu, 24 Jun 2010, Janning wrote:
> > We have a 12 GB RAM machine with intel i7-975 and using
> > 3 disks "Seagate Barracuda 7200.11, ST31500341AS (1.5 GB)"
>
> Those discs are 1.5TB, not 1.5GB.

sorry, my fault.

> > One disk for the system and WAL etc. and one SW RAID-0 with two disks for
> > postgresql data. Our database is about 24GB.
>
> Beware of RAID-0 - make sure you can recover the data when (not if) a disc
> fails.

oh sorry again, its a raid-1 of course. shame on me.

> > Our munin graph reports at 9:00 a clock writes of 3000 blocks per second
> > and reads of about 1000 blocks per second on our disk which holds the
> > data directories of postgresql (WAL are on a different disk)
> >
> > 3000 blocks ~ about 3 MB/s write
> > 1000 blocks ~ about 1 MB/s read
> >
> > At the same time we have nearly 50% CPU I/O wait and only 12% user CPU
> > load (so 4 of 8 cpu cores are in use for io wait)
>
> Not quite sure what situation you are measuring these figures under.
> However, as a typical figure, let's say you are doing random access with
> 8kB blocks (as in Postgres), and the access time on your drive is 8.5ms
> (as with these drives).
>
> For each drive, you will be able to read/write approximately 8kB /
> 0.0085s, giving 941kB per second. If you have multiple processes all doing
> random access, then you may be able to utilise both discs and get double
> that.

So with your calculation I have a maximum of 2MB/s random access. So i really 
need to upgrade my disk configuration!

> > Of course, writing large chunks is quite a different usage pattern. But I
> > am wondering that writing 3MB/s and reading 1 MB/s seams to be a limit if
> > i can run a test with 89 MB/s writing and 110MB/s reading.
>
> That's quite right, and typical performance figures for a drive like that.

thanks for your help.

kind regards 
Janning

> Matthew
>
> --
>  Don't criticise a man until you have walked a mile in his shoes; and if
>  you do at least he will be a mile behind you and bare footed.