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Re: concurrent IO in postgres?

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: wozniak(at)lanl(dot)gov
Cc: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: concurrent IO in postgres?
Date: 2010-12-26 01:48:16
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Lists: pgsql-performance
On Thu, Dec 23, 2010 at 11:46 AM, Przemek Wozniak <wozniak(at)lanl(dot)gov> wrote:

> In one test I was running between 1 and 32 clients simultaneously
> writing lots of data using copy binary.

Are you by-passing WAL?  If not, you are likely serializing on that.
Not so much the writing, but the lock.

> 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.

But those "writes" are probably just copies of 8K into kernel's RAM,
and so very fast.

> So the actual IO is effectively serialized by the backend.

If the background writer cannot keep up, then the individual backends
start doing writes as well, so it isn't really serialized..



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