Message-ID: <4510F720.5010007@logix-tt.com>
Date: Wed, 20 Sep 2006 10:09:04 +0200
From: Markus Schaber <schabi@logix-tt.com>
Organization: Logical Tracking and Tracing International AG, Switzerland
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To: pgsql-performance@postgresql.org
Subject: Re: Large tables (was: RAID 0 not as fast as
References: <C134B9B9.31741%llonergan@greenplum.com>
In-Reply-To: <C134B9B9.31741%llonergan@greenplum.com>
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Hi, Luke,

Luke Lonergan wrote:

> Since PG's heap scan is single threaded, the seek rate is equivalent to a
> single disk (even though RAID arrays may have many spindles), the typical
> random seek rates are around 100-200 seeks per second from within the
> backend.  That means that as sequential scan performance increases, such as
> happens when using large RAID arrays, the random_page_cost will range from
> 50 to 300 linearly as the size of the RAID array increases.

Do you think that adding some posix_fadvise() calls to the backend to
pre-fetch some blocks into the OS cache asynchroneously could improve
that situation?

I could imagine that e. G. index bitmap scans could profit in the heap
fetching stage by fadvise()ing the next few blocks.

Maybe asynchroneous I/O could be used for the same benefit, but
posix_fadvise is less() intrusive, and can easily be #define'd out on
platforms that don't support it.

Combine this with the Linux Kernel I/O Scheduler patches (readahead
improvements) that were discussed here in summer...

Regards,
Markus

-- 
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

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