Subject: Re: [Ext2-devel] Re: [PATCH 2/3] ext3 reservation allow turn off
	for specifed file
From: Mingming Cao <cmm@us.ibm.com>
To: Andrew Morton <akpm@osdl.org>
Cc: ray-lk@madrabbit.org, sct@redhat.com, pbadari@us.ibm.com,
       linux-kernel@vger.kernel.org, ext2-devel@lists.sourceforge.net
In-Reply-To: <20041026131842.45b99834.akpm@osdl.org>
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Date: 26 Oct 2004 16:01:25 -0700
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On Tue, 2004-10-26 at 13:18, Andrew Morton wrote:
> Mingming Cao <cmm@us.ibm.com> wrote:
> >
> > > I wonder how important this optimisation really is?  I bet no applications
> >  > are using posix_fadvise(POSIX_FADV_RANDOM) anyway.
> >  > 
> >  I don't know if there is application using the POSIX_FADV_RANDOM. No? If
> >  this is the truth, I think we don't need this optimization at present.
> >  Logically reservation does not benefit seeky random write, but there is
> >  no benchmark showing performance issue so far. We have already provided
> >  ways for applications turn off reservation through the existing ioctl
> >  for specified file and -o noreservation mount option for the whole
> >  filesystem.
> 
> Well we definitely don't want to be encouraging application developers to be
> adding ext3-specific ioctls.  So we need to work out if any applications
> can get significant benefit from manually disabling reservations and if
> so, wire up fadvise() into filesystems and do it that way.
> 
Okey. Also, if there is such a need to disable reservation, maybe we
could think about closing the reservation window dynamically based on
past I/O pattern, when user application did not give any hint.

> Do you know if disabling reservations helps any workloads?
> 
Not that I aware of.  We have done several microbenchmark on sequential
and random workload, haven't seen regression with reservations so far.
But our testing is certainly not comprehensive.

Mingming

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