Date: Mon, 8 Jan 2007 18:18:12 -0800
From: Jeremy Higdon <jeremy@sgi.com>
To: Paul Jackson <pj@sgi.com>
Cc: Jeff Garzik <jeff@garzik.org>, tytso@mit.edu, suparna@in.ibm.com,
       akpm@osdl.org, w@1wt.eu, torvalds@osdl.org, hpa@zytor.com,
       git@vger.kernel.org, nigel@nigel.suspend2.net, warthog9@kernel.org,
       randy.dunlap@oracle.com, pavel@ucw.cz, linux-kernel@vger.kernel.org,
       webmaster@kernel.org, linux-ext4@vger.kernel.org
Subject: Re: How git affects kernel.org performance
Message-ID: <20070109021812.GE44262@sgi.com>
References: <1168140954.2153.1.camel@nigel.suspend2.net> <45A08269.4050504@zytor.com> <45A083F2.5000000@zytor.com> <Pine.LNX.4.64.0701062130260.3661@woody.osdl.org> <20070107085526.GR24090@1wt.eu> <20070107011542.3496bc76.akpm@osdl.org> <20070108030555.GA7289@in.ibm.com> <20070108125819.GA32756@thunk.org> <45A24A65.1070706@garzik.org> <20070108170934.dafc5b81.pj@sgi.com>
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On Mon, Jan 08, 2007 at 05:09:34PM -0800, Paul Jackson wrote:
> Jeff wrote:
> > Something I just thought of:  ATA and SCSI hard disks do their own
> > read-ahead.
> 
> Probably this is wishful thinking on my part, but I would have hoped
> that most of the read-ahead they did was for stuff that happened to be
> on the cylinder they were reading anyway.  So long as their read-ahead
> doesn't cause much extra or delayed disk head motion, what does it
> matter?


And they usually won't readahead if there is another command to
process, though they can be set up to read unrequested data in
spite of outstanding commands.

When they are reading ahead, they'll only fetch LBAs beyond the last
request until a buffer fills or the readahead gets interrupted.

jeremy
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