From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:60335) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QqizF-0006mB-Dw for qemu-devel@nongnu.org; Tue, 09 Aug 2011 05:52:38 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QqizE-0002yD-8g for qemu-devel@nongnu.org; Tue, 09 Aug 2011 05:52:37 -0400 Received: from e28smtp08.in.ibm.com ([122.248.162.8]:42766) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QqizD-0002xy-I1 for qemu-devel@nongnu.org; Tue, 09 Aug 2011 05:52:36 -0400 Received: from d28relay03.in.ibm.com (d28relay03.in.ibm.com [9.184.220.60]) by e28smtp08.in.ibm.com (8.14.4/8.13.1) with ESMTP id p799S5na003815 for ; Tue, 9 Aug 2011 14:58:05 +0530 Received: from d28av05.in.ibm.com (d28av05.in.ibm.com [9.184.220.67]) by d28relay03.in.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p799qUR43604610 for ; Tue, 9 Aug 2011 15:22:30 +0530 Received: from d28av05.in.ibm.com (loopback [127.0.0.1]) by d28av05.in.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p799qUnG012746 for ; Tue, 9 Aug 2011 19:52:30 +1000 Message-ID: <4E40FEBA.60001@in.ibm.com> Date: Tue, 09 Aug 2011 15:02:42 +0530 From: supriya kannery MIME-Version: 1.0 References: <4E3BB203.5060703@redhat.com> <20110805131247.GC6201@redhat.com> <20110805142812.GB17323@lst.de> <4E3C0FAA.2080303@redhat.com> <4E3C1117.8000009@codemonkey.ws> <4E3F89FC.8090109@linux.vnet.ibm.com> <4E3F9A7E.1090906@redhat.com> <4E40FC59.7010104@in.ibm.com> <4E410306.4010002@redhat.com> In-Reply-To: <4E410306.4010002@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Safely reopening image files by stashing fds List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: supriyak@linux.vnet.ibm.com, Stefan Hajnoczi , qemu-devel , Paolo Bonzini , Christoph Hellwig 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 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