On Mon, Aug 29, 2005 at 09:42:46AM -0400, Rémy Beaumont wrote:
We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3.

Nope, it's an IO bottleneck.

The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s

That's the sign of an IO bottleneck.

The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads. So my question is where is this iowait time spent ?

Waiting for the seeks. postgres doesn't do async io, so it requests a
block, waits for it to come in, then requests another block, etc. The
utilization on the netapp isn't going to be high because it doesn't have
a queue of requests and can't do readahead because the IO is random. The
only way to improve the situation would be to reduce the latency of the
seeks. If I read the numbers right you're only getting about 130
seeks/s, which ain't great. I don't know how much latency the netapp
adds in the this configuration; have you tried benchmarking
direct-attach disks?

Mike Stone

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