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Date: Wed, 29 Jun 2011 13:52:19 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] Default cache mode
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To: Kevin Wolf <kwolf@redhat.com>
Cc: 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 12:59 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> I'm not entirely sure if I should suggest writeback or none as the new
> default, but I think it could make sense to change it.

I agree that cache=none is safe and fast with correct guests and local
disks.  It is beaten by cache=writeback in cases where the host has a
lot of buffer cache to play with and the guest has very little - for
example when running qemu without a -m argument.  If I was running
qemu-system-arm I might like cache=writeback.  If I was virtualizing
x86 servers I might like cache=none.

cache=none as a default - which some management tools inherit - would
be reasonable.  Folks using command-line qemu can always tweak to use
cache=writeback or even cache=unsafe if performance matters but data
integrity does not.

Whether it is safe to transition to cache=none or not depends on what
broken guests are still widely deployed.  Does CentOS 5 flush the disk
cache?

Stefan