On Sun, Jun 19, 2011 at 5:02 PM, Dor Laor <dl...@redhat.com> wrote: > On 06/18/2011 12:17 PM, Stefan Hajnoczi wrote: >> >> On Sat, Jun 18, 2011 at 10:15 AM, Stefan Hajnoczi<stefa...@gmail.com> >> wrote: >>> >>> On Fri, Jun 17, 2011 at 1:31 PM, Marcelo Tosatti<mtosa...@redhat.com> >>> 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? > - 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? This approach makes sense to me. Stefan
