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Date: Tue, 17 May 2011 17:41:26 +0200
From: Christoph Hellwig <hch@lst.de>
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References: <20110516181023.7142.33402.sendpatchset@skannery>
	<20110516202354.GA8955@lst.de> <4DD192BD.3030200@codemonkey.ws>
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Subject: Re: [Qemu-devel] [RFC Patch 0/3]Qemu: Enable dynamic cache
	change	through qemu monitor
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Supriya Kannery <supriyak@linux.vnet.ibm.com>, Prerna Saxena <prerna@linux.vnet.ibm.com>, Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org

On Mon, May 16, 2011 at 04:10:21PM -0500, Anthony Liguori wrote:
> To further clarify:
>
> Today cache=none|writethrough|writeback does two things.  It:
>
> 1) Changes the WCE flag that's visible to the guest
>
> 2) Determines whether the host page cache is used for doing guest I/O
>
> As Christoph is very correct in pointing out, we cannot change (1) at run 
> time because this is guest visible.  You will break a guest if you do this.
>
> But it's still desirable to be able to change (2) at run time.  Before we 
> can do this properly though, we need to separate out the logic for setting 
> (1) vs. (2).
>
> And ideally, we would allow (1) to be changed by the guest itself at run 
> time which allows for full dynamic control.  This is what he's referring to 
> below.

Exactly.  Setting/clearing the BDRV_O_NO_FLUSH also seems useful, maybe
in addition to also allowing an equivalent for the writethrough modes.