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Date: Wed, 29 Jun 2011 14:57:17 +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: Alexander Graf <agraf@suse.de>
Cc: Kevin Wolf <kwolf@redhat.com>, Qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Christoph Hellwig <hch@lst.de>

On Wed, Jun 29, 2011 at 1:21 PM, Alexander Graf <agraf@suse.de> wrote:
> On 29.06.2011, at 13:59, Kevin Wolf 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.
>
> None as default would be a bad choice, as not all underlying file systems support it. Try cache=none on tmpfs or nfs. It will just fail on you.
> What I'd personally like to see is some sort of detection that can determine which caching mode would perform best. So it would do cache=none when it makes sense (raw block device), otherwise cache=writeback.

O_DIRECT works on the Linux NFS client.  The performance currently
isn't great with vectored I/O (which QEMU does) but there is work
ongoing to improve it.

Your point makes sense though.  The default (no explicit cache=)
should mean "the most appropriate caching mode".

Stefan