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Date: Thu, 14 Jan 2010 21:18:54 +0100
From: "fkater@googlemail.com" <fkater@googlemail.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Inserting 8MB bytea: just 25% of disk perf used?
Message-ID: <20100114201854.GC2384@comppasch2>
References: <20100114142903.GB2973@comppasch2>
	<alpine.DEB.2.00.1001141458590.6195@aragorn.flymine.org>
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Thanks a lot for the detailed reply.

Matthew Wakeling:

> On Thu, 14 Jan 2010, fkater@googlemail.com wrote:
> > This takes about 50s, so, 800MB/50s = 16MB/s.
> >
> > However the harddisk (sata) could write 43 MB/s in the worst
> > case! Why is write performance limited to 16 MB/s?
> 
> Several reasons:
> 
> The data needs to be written first to the WAL, in order to provide 
> crash-safety. So you're actually writing 1600MB, not 800.

I understand. So the actual throughput is 32MB/s which is
closer to 43 MB/s, of course.

Can I verify that by temporarily disabling WAL writes
completely and see if the thoughput is then doubled?

 Felix