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Date: Wed, 09 Jan 2013 20:16:21 +0800
From: Liu Yuan <namei.unix@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

On 01/09/2013 08:07 PM, Liu Yuan wrote:
> and my proposal (adding another field to BlockDriverState to allow
> self-interpretation cache flag) can work well with current block layer:
> 
> Sheepdog driver behavior:
>    cache flags            WCE toggled and resulting behavior
>    writethrough           writethrough
>    writeback              writetrhough (writeback + flush)
> 
> We can see that the writeback behavior for Guest-WCE toggling is the
> same as expected. The difference is that if we set cache=writethrough,
> guest can't change it via WCE toggling.

I am wondering if above scheme can apply to the block layer, then
'cache=xxx' can act as a stronger control over WCE toggling. Then we
don't need extra field.

Thanks,
Yuan