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

[supriya kannery <supriyak@in.ibm.com>]

Kevin Wolf wrote:
> Am 09.08.2011 11:22, schrieb supriya kannery:
>   
>> Kevin Wolf wrote:
>>     
>>> 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.
>>>
>>>   
>>>       
>> Could you please explain "In the end we want it to use bdrv_reopen" at 
>> bit more.
>> When fcntl() can change O_DIRECT on  open fd , is there a  need to 
>> "re-open"
>> the image file?
>>     
>
> What I meant is that in the end, with a generic bdrv_reopen(), we can
> have raw-posix only call dup() and fcntl() instead of doing a
> close()/open() sequence if it can satisfy the new flags this way. But
> this would be an implementation detail and not be visible in the interface.
>
> Kevin
>   

ok
- thanks, Supriya