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Date: Tue, 31 Jan 2012 21:35:22 +0000
From: Brian Candler <B.Candler@pobox.com>
Subject: Re: Performance problem - reads slower than writes
Message-ID: <20120131213522.GA47420@nsrc.org>
References: <20120130220019.GA45782@nsrc.org> <20120131020508.GF9090@dastard>
	<20120131103126.GA46170@nsrc.org> <20120131200635.GI9090@dastard>
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To: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com

On Wed, Feb 01, 2012 at 07:06:35AM +1100, Dave Chinner wrote:
> The number of IOs does not equal the number of seeks. Two adjacent,
> sequential IOs issued serially will show up as two IOs, even though
> there was no seek in between. Especially if the files are large
> enough that readahead tops out (500-800k is large enough for this as
> readahead maximum is 128k by default).  So it might be taking 3-4
> IOs just to read the file data.

Ah. And if the IOs are not stacked up, then the platter has to rotate nearly
a whole turn to perform the next one.

> > So the next thing I'd have to do is to try to get a trace of the I/O
> > operations being performed, and I don't know how to do that.
> 
> blktrace/blkparse or seekwatcher.

Excellent, just what I wanted. I've made a start with this and will report
back.

Many thanks for the help and pointers you have provided.

Regards,

Brian.

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