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Date: Fri, 05 Aug 2011 17:43:38 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] Safely reopening image files by stashing fds
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Supriya Kannery <supriyak@linux.vnet.ibm.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>, Christoph Hellwig <hch@lst.de>

Am 05.08.2011 17:24, schrieb Stefan Hajnoczi:
> On Fri, Aug 5, 2011 at 3:28 PM, Christoph Hellwig <hch@lst.de> wrote:
>> On Fri, Aug 05, 2011 at 02:12:48PM +0100, Daniel P. Berrange wrote:
>>>> Because you cannot change O_DIRECT on an open fd :(.  This is why
>>>> we're going through this pain.
>>>
>>> Hmm, I remember hearing that before, but looking at the current fcntl()
>>> manpage, it claims you *can* change O_DIRECT using SET_FL. Perhaps this
>>> is a newish feature, but it'd be nicer to use it if possible ?
>>
>> It's been there since day 1 of O_DIRECT support.
> 
> Sorry, my bad.  So for Linux we could just use fcntl for
> block_set_hostcache and not bother with reopening.  However, we will
> need to reopen should we wish to support changing O_DSYNC.

We do wish to support that.

Anthony thinks that allowing the guest to toggle WCE is a prerequisite
for making cache=writeback the default. And this is something that I
definitely want to do for 1.0.

Kevin