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Date: Mon, 1 Feb 2010 10:27:56 +0100
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] Re: [PATCH] block: Enable fall-back to read-only for
	backing file
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References: <4B65B4B7.2030700@redhat.com> <4B6694C8.4010703@redhat.com>
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To: Alexander Graf <agraf@suse.de>
Cc: Kevin Wolf <kwolf@redhat.com>, Naphtali Sprei <nsprei@redhat.com>, Christoph Hellwig <hch@lst.de>, Sheng Yang <sheng@linux.intel.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On Mon, Feb 01, 2010 at 10:25:52AM +0100, Alexander Graf wrote:
> We don't know beforehand if a user will use the commit comnand during  
> tje runtime of the vm.
> 
> IMHO it'd be best to always open backing files read only and try to  
> open them for write access on the commit command. That command can  
> then fail gaciously.

That's another option.  Basically this means opening another
BlockDriverState for the backing device that is writeable inside
bdrv_commit().