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Date: Sat, 16 Apr 2011 09:46:38 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH] raw-posix: Linearize direct I/O on Linux
	NFS
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To: Anthony Liguori <aliguori@us.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org, Khoa Huynh <khoa@us.ibm.com>, pbadari@linux.vnet.ibm.com, Christoph Hellwig <hch@lst.de>

On Sat, Apr 16, 2011 at 12:00 AM, Anthony Liguori <aliguori@us.ibm.com> wro=
te:
> 3) We have no way to detect when we no longer need a work around which ma=
kes
> (2) really unappealing.

I agree.

> 4) That leaves us with:
> =A0 =A0a) waiting for NFS to get fixed properly and just living with wors=
e
> performance on older kernels
>
> =A0 =A0b) having a user-tunable switch to enable bouncing
>
> I really dislike the idea of (b) because we're stuck with it forever and
> it's yet another switch for people to mistakenly depend on.

The user-tunable switch is potentially interesting for performance
troubleshooting.  We have seen another file system which has issues
with vectored direct I/O.  It would have been much easier to identify
the problem by telling the user "Try running it with linearize=3Don and
see if it makes a difference".

But let's try harder on linux-nfs.

Stefan