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Date: Wed, 29 Jun 2011 15:55:45 +0200
From: Christoph Hellwig <hch@lst.de>
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References: <4E0B1399.4050807@redhat.com> <4E0B15B2.40504@codemonkey.ws>
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Subject: Re: [Qemu-devel] Default cache mode
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Avi Kivity <avi@redhat.com>, Christoph Hellwig <hch@lst.de>, Qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

On Wed, Jun 29, 2011 at 07:08:18AM -0500, Anthony Liguori wrote:
> As long as we advertise wce and wce can be toggled from the guest, I don't 
> think the default is all that important.  I think cache=on is the right 
> default for most common use cases.

What do you mean with cache=on?  We have
cache=[none,writeback,writethrough,unsafe].  As discussed about a dozen
times before these are an incomplete choice of the [odirect=on,off;
osync=on,off, ignore cache flushes=on,off] matrix.

Not using O_DIRECT when it is available is almost always the wrong choice.
It means an additional data copy, VM pressure and exercise of often suck
fsync/O_SYNC codepatheses, with only a benefit if you have multiple VMs
actually using the same dataset with the same COW backing image.

Using WCE = 0 or 1 is a tradeoff of safety vs performance.  WCE=0 performance
generally sucks badly, but offers data integrity for older or non-PCish host
operating systems.