From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:38959) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QaonM-0000qv-HD for qemu-devel@nongnu.org; Sun, 26 Jun 2011 08:50:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QaonJ-00087D-HP for qemu-devel@nongnu.org; Sun, 26 Jun 2011 08:50:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:13995) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QaonJ-000873-7x for qemu-devel@nongnu.org; Sun, 26 Jun 2011 08:50:33 -0400 Message-ID: <4E072B0B.4010105@redhat.com> Date: Sun, 26 Jun 2011 15:50:19 +0300 From: Dor Laor MIME-Version: 1.0 References: <1308075511-4745-1-git-send-email-stefanha@linux.vnet.ibm.com> <4DF9F899.5050301@redhat.com> <4DF9FBE4.9080300@redhat.com> <4DFA004E.9010001@redhat.com> <20110616145243.GB12173@amt.cnet> <20110616153018.GA20714@stefanha-thinkpad.localdomain> <20110617123152.GA7379@amt.cnet> <4DFE1D8C.3000402@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Image streaming and live block copy Reply-To: dlaor@redhat.com List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Anthony Liguori , Stefan Hajnoczi , jes sorensen , Marcelo Tosatti , qemu-devel@nongnu.org, Avi Kivity , Adam Litke On 06/24/2011 12:28 PM, Stefan Hajnoczi wrote: > On Sun, Jun 19, 2011 at 5:02 PM, Dor Laor wrote: >> On 06/18/2011 12:17 PM, Stefan Hajnoczi wrote: >>> >>> On Sat, Jun 18, 2011 at 10:15 AM, Stefan Hajnoczi >>> wrote: >>>> >>>> On Fri, Jun 17, 2011 at 1:31 PM, Marcelo Tosatti >>>> wrote: >>>>> >>>>> On Thu, Jun 16, 2011 at 04:30:18PM +0100, Stefan Hajnoczi wrote: >>>>>> >>>>>> On Thu, Jun 16, 2011 at 11:52:43AM -0300, Marcelo Tosatti wrote: >>>>>> This approach does not use the backing file feature? >>>>>> >>>>>>> blkstream block driver: >>>>>>> >>>>>>> - Maintain in memory whether given block is allocated in local image, >>>>>>> if not, read from remote, write to local. Set block as local. >>>>>>> Local and remote simply two block drivers from image streaming driver >>>>>>> POV. >>>>>>> - Once all blocks are local, notify mgmt so it can switch to local >>>>>>> copy. >>>>>>> - Writes are mirrored to source and destination, minding guest writes >>>>>>> over copy writes. >>>>>> >>>>>> We open the remote file read-only for image streaming and do not want >>>>>> to >>>>>> mirror writes. >>>>> >>>>> Why not? Is there any disadvantage of mirroring writes? >>>> >>>> Think of the use case with a Fedora master image over NFS. You want a >>>> local clone of that master image and use the stream command to copy >>>> the data from the master image into the local clone. >>>> >>>> You cannot modify that master image because other VMs are using it too >>>> and/or you want to be able to clone new VMs from it in the future. >>> >>> BTW the workaround is to create two local images: >>> 1. Local clone with master image as a backing file. This is the live >>> block copy source image. >>> 2. Local image without a backing file. This is the live block copy >>> destination image. >>> >>> But this is not very elegant. Writes get mirrored so that crash recovery >>> works. >> >> There is an easier work around for image streaming using live block copy >> (mirror approach): >> - Create the dst VM as an empty new COW image of the src (even over >> the non shared storage, use some protocol tag for the src location >> like nbd://original_path/src_file_name) > > Migration and non-shared storage has come up a few times in this > discussion. But both live block copy and image streaming need access > to source and destination - they do not have explicit non-shared > storage support. I think non-shared and using nbd:// is orthogonal to > the discussion. Just want to check that you agree and I haven't > missed something? You're right, I was mainly trying to be as general as possible. > >> - Run the usual live block copy of src image (master read only OS >> template) to the destination. >> - Use a -src-read-only flag that will make the copy skip the src >> writing. >> >> Voila - no duplicate writes, crash recovery works since we reference the >> original image and we share the code. > > So the running guest is using the destination image since the source > is read-only? Yes. The image will run on the destination not because of the source in RO state but because that is what we look for. > > This approach makes sense to me. > > Stefan > > >