Subject: Re: Huge Data sets, simple queries
From: "Jeffrey W. Baker" <jwbaker@acm.org>
To: Luke Lonergan <llonergan@greenplum.com>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <C0058CC2.1B6B1%llonergan@greenplum.com>
References: <C0058CC2.1B6B1%llonergan@greenplum.com>
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Date: Wed, 01 Feb 2006 00:25:13 -0800
Message-Id: <1138782313.14732.1.camel@noodles>
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On Tue, 2006-01-31 at 21:53 -0800, Luke Lonergan wrote:
> Jeffrey,
> 
> On 1/31/06 8:09 PM, "Jeffrey W. Baker" <jwbaker@acm.org> wrote:
> >> ... Prove it.
> > I think I've proved my point.  Software RAID1 read balancing provides
> > 0%, 300%, 100%, and 100% speedup on 1, 2, 4, and 8 threads,
> > respectively.  In the presence of random I/O, the results are even
> > better.
> > Anyone who thinks they have a single-threaded workload has not yet
> > encountered the autovacuum daemon.
> 
> Good data - interesting case.  I presume from your results that you had to
> make the I/Os non-overlapping (the "skip" option to dd) in order to get the
> concurrent access to work.  Why the particular choice of offset - 3.2GB in
> this case?

No particular reason.  8k x 100000 is what the last guy used upthread.
> 
> So - the bandwidth doubles in specific circumstances under concurrent
> workloads - not relevant to "Huge Data sets, simple queries", but possibly
> helpful for certain kinds of OLTP applications.

Ah, but someday Pg will be able to concurrently read from two
datastreams to complete a single query.  And that day will be glorious
and fine, and you'll want as much disk concurrency as you can get your
hands on.

-jwb