Re: [Qemu-devel] Safely reopening image files by stashing fds

[Kevin Wolf <kwolf@redhat.com>]

Am 08.08.2011 09:02, schrieb Supriya Kannery:
> On 08/05/2011 09:19 PM, Anthony Liguori wrote:
>> On 08/05/2011 10:43 AM, Kevin Wolf wrote:
>>> 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.
>>
>> Indeed.
>>
> 
> We discussed the following so far...
> 1. How to safely reopen image files
> 2. Dynamic hostcache change
> 3. Support for dynamic change of O_DSYNC
> 
> Since 2 is independent of 1, shall I go ahead implementing
> hostcache change using fcntl.
> 
> Implementation for safely reopening image files using "BDRVReopenState"
> can be done separately as a pre-requisite before implementing 3

Doing it separately means that we would introduce yet another callback
that is used just to change O_DIRECT. In the end we want it to use
bdrv_reopen(), too, so I'm not sure if there is a need for a temporary
solution.

Actually, once we know what we really want (I haven't seen many comments
on the BDRVReopenState suggestion yet), it should be pretty easy to
implement.

Kevin