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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
Message-ID: AANLkTimhWg9b5uE6aeog0X2rurCieF6fFF4h61dm=fpq@mail.gmail.com (view raw or flat)
Thread:
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..

Cheers,

Jeff

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