Message-ID: <439750A3.2030805@redhat.com>
Date: Wed, 07 Dec 2005 16:14:11 -0500
From: Peter Staubach <staubach@redhat.com>
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To: Kenny Simpson <theonetruekenny@yahoo.com>
CC: linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: nfs question - ftruncate vs pwrite
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Kenny Simpson wrote:

>Sorry about the previous partial message...
>
>If a file is extended via ftruncate, the new empty pages are read in before the the ftruncate
>returns (taking 64mS on my machine), but if the file is extended via pwrite, nothing is read in
>and the system call is very quick (34uS).
>
>Why is there such a difference?  Is there another cheap way to grow a file and map in its new
>pages?  Am I missing some other semantic difference between ftruncate and a pwrite past the end of
>the file?
>

You might use tcpdump or etherreal to see what the different traffic looks
like.  I suspect that ftruncate() leads a SETATTR operation while pwrite()
leads to a WRITE operation.

       ps
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