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<h...@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
