From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:44744) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Qqipa-00047r-50 for qemu-devel@nongnu.org; Tue, 09 Aug 2011 05:42:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QqipY-0008Tg-Js for qemu-devel@nongnu.org; Tue, 09 Aug 2011 05:42:38 -0400 Received: from e23smtp02.au.ibm.com ([202.81.31.144]:52640) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QqipX-0008Sp-V7 for qemu-devel@nongnu.org; Tue, 09 Aug 2011 05:42:36 -0400 Received: from d23relay04.au.ibm.com (d23relay04.au.ibm.com [202.81.31.246]) by e23smtp02.au.ibm.com (8.14.4/8.13.1) with ESMTP id p799a9bS001085 for ; Tue, 9 Aug 2011 19:36:09 +1000 Received: from d23av03.au.ibm.com (d23av03.au.ibm.com [9.190.234.97]) by d23relay04.au.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p799fSoN1499326 for ; Tue, 9 Aug 2011 19:41:29 +1000 Received: from d23av03.au.ibm.com (loopback [127.0.0.1]) by d23av03.au.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p799gOwY005838 for ; Tue, 9 Aug 2011 19:42:24 +1000 Message-ID: <4E40FC59.7010104@in.ibm.com> Date: Tue, 09 Aug 2011 14:52:33 +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> In-Reply-To: <4E3F9A7E.1090906@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 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? Considering the current way of having separate high level commands for changing block parameters (block_set_hostcache, and may be block_set_flush in furture), these dynamic requests will be sequential. So wouldn't it be better to avoid re-opening of image if possible for individual flag change request that comes in? > 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 > Will work on to get an RFC patch with this proposed BDRVReopenState to get more inputs.