Re: concurrent IO in postgres?

On Thu, 2010-12-23 at 11:24 -0700, Scott Marlowe wrote:
> On Thu, Dec 23, 2010 at 10:37 AM, Przemek Wozniak <wozniak@lanl.gov> wrote:
> > When testing the IO performance of ioSAN storage device from FusionIO
> > (650GB MLC version) one of the things I tried is a set of IO intensive
> > operations in Postgres: bulk data loads, updates, and queries calling
> > for random IO. So far I cannot make Postgres take advantage of this
>
> So, were you running a lot of these at once?  Or just single threaded?

> I get very good io concurrency with lots of parallel postgresql
> connections on a 34 disk SAS setup with a battery backed controller.

In one test I was running between 1 and 32 clients simultaneously
writing lots of data using copy binary. The problem is that with a large
RAM buffer it all goes there, and then the background writer, a single
postgres process, will issue write requests one at a time I suspect.
So the actual IO is effectively serialized by the backend.