On Fri, May 25, 2012 at 11:25 PM, Paolo Bonzini <pbonz...@redhat.com> wrote: > Il 25/05/2012 14:09, Stefan Hajnoczi ha scritto: >>> > >>> > Perhaps that be simply a new qemu-img subcommand? It should be possible >>> > to run it while the VM is offline. Then the file that is produced could >>> > be fed to blockdev-dirty-enable. >> For both continuous replication and incremental backups we cannot >> require that the guest is shut down in order to collect the dirty >> bitmap, I think. > > Yes, that is a problem for internal snapshots. For external snapshots, > see the drive-mirror command's sync parameter. Perhaps we can add a > blockdev-dirty-fill command that adds allocated sectors up to a given > base to the dirty bitmap. > >> I think we really need a libvirt API because a local file not only has >> permissions issues but also is not network transparent. The continuous >> replication server runs on another machine, how will it access the dirty >> bitmap file? > > This is still using a "push" model where the dirty data is sent from > QEMU to the replication server, so the dirty bitmap is not needed on the > machine that runs the replication server---only on the machine that runs > the VM (to preserve the bitmap across VM shutdowns including power > loss). It has to be stored on shared storage if you plan to run the VM > from multiple hosts.
Why reinventing the wheel? Wouldnt it be much better to externalize the snapshotting. Some/Many filesystems support snapshotting today. A-whole-lot-of/most-non-consumer-grade block storage devices support it too. So a different way to do this would be to use a mechanism to quiescence the backing file/device and then call out to an external agent to snapshot the backing file or the backing device. Other external tools can then be used to compute a dense delta between this new snapshot and the previous snapshot and transfer to the other side. I think all filesystems that support snapshotting on file level support APIs for a cheap way to cumpute the block deltas. I would imagine all midrange or better block storage devices that support LUN snapshotting provide this too. So why do snapshotting and computation of snapshot deltas in qemu ? Why not just externalize it with "you want snapshotting and incremental replication => you must use a system where file/block supports it". regards ronnie sahlberg
