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From: Brian Candler <B.Candler@pobox.com>
To: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com
Subject: Re: Performance problem - reads slower than writes
Date: Tue, 31 Jan 2012 21:35:22 +0000	[thread overview]
Message-ID: <20120131213522.GA47420@nsrc.org> (raw)
In-Reply-To: <20120131200635.GI9090@dastard>

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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      reply	other threads:[~2012-01-31 21:35 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-30 22:00 Performance problem - reads slower than writes Brian Candler
2012-01-31  2:05 ` Dave Chinner
2012-01-31 10:31   ` Brian Candler
2012-01-31 14:16     ` Brian Candler
2012-01-31 20:25       ` Dave Chinner
2012-02-01  7:29         ` Stan Hoeppner
2012-02-03 18:47         ` Brian Candler
2012-02-03 19:03           ` Christoph Hellwig
2012-02-03 21:01             ` Brian Candler
2012-02-03 21:17               ` Brian Candler
2012-02-05 22:50                 ` Dave Chinner
2012-02-05 22:43               ` Dave Chinner
2012-01-31 14:52     ` Christoph Hellwig
2012-01-31 21:52       ` Brian Candler
2012-02-01  0:50         ` Raghavendra D Prabhu
2012-02-01  3:59         ` Dave Chinner
2012-02-03 11:54       ` Brian Candler
2012-02-03 19:42         ` Stan Hoeppner
2012-02-03 22:10           ` Brian Candler
2012-02-04  9:59             ` Stan Hoeppner
2012-02-04 11:24               ` Brian Candler
2012-02-04 12:49                 ` Stan Hoeppner
2012-02-04 20:04                   ` Brian Candler
2012-02-04 20:44                     ` Joe Landman
2012-02-06 10:40                       ` Brian Candler
2012-02-07 17:30                       ` Brian Candler
2012-02-05  5:16                     ` Stan Hoeppner
2012-02-05  9:05                       ` Brian Candler
2012-01-31 20:06     ` Dave Chinner
2012-01-31 21:35       ` Brian Candler [this message]

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