Re: Write performance

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.

> 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.

> 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.

> 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.

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.