Hello Jagane, My original idea was this: Have a data disk implemented as an LV. Use drbd or dm-replicator to asynchronously replicate the vg to a remote location. When I make a snapshot of the LV, the snapshot will be replicated along with it. Have an application quiesce itself and make the LV snapshot (doing it from within a vm is also possible, but a bit more challenging) on the source site. This means there is an application consistent snapshot available. Problem is the lvm administration need also to be replicated to the second site. Quite difficult I think.
Another idea, not using lvm but qcow2, is to create a snapshot in a separate file from the master file. The snapshot file holds the changed blocks. Then, when you create a second snapshot, the changes from that point onwards will be placed in the second snapshot, making the first one static. The first one can be used for replication. When that is done, it can be deleted. A new snapshot is created and will function as the new target for changed blocks and the snapshot before that one will be replicated and so on. This means 2 snapshot files are needed to make this work. The snapshots on the other site will be stored next to a copy of the master file. When needed a vm can be started on any of these snapshots. Guess what, the NetApp SnapMirror mechanism also uses two snapshot's to make it work. I know NetApp and LVM but am rather new to qcow2, so i am not sure if i'm suggesting anything remotely sane here, so forgive me that's the case. Greetz, Fred On Sun, May 15, 2011 at 7:16 PM, Jagane Sundar <jag...@sundar.org> wrote: > Hello Fred, > > As Stefan points out, there is a lot of interest in such functionality. > > I am the author of a feature called Livebackup - which is aimed at > solving exactly this problem - backup of live virtual machines. The > additional constraint I place is that backup of the VM should be > enabled regardless of the type of storage in use - file on local disk, > file on NAS, LVM logical volume on local disk, LVM volume on an > iSCSI volume. > > More information is at: > http://wiki.qemu.org/Features/Livebackup > > I investigated the approach that you outline below - LVM snapshot > followed by transfer of the blocks out of the VM host. The problem > I ran into was that there was no easy way to identify the blocks that > were modified since the last backup was taken. To the best of my > knowledge, rsync uses checksums of entire files to determine whether > a file was modified or not - in our use case the whole virtual disk > is one single file, do rsync will not work well. > > One of the 'features' of Livebackup is that I chose to invent my own > rudimentary protocol for transferring the blocks out of the VM > host. iSCSI seems like a standard protocol that would suit the > purpose; however it does not fit well with the architecture of > Livebackup - I would probably need to implement an iSCSI target > function inside Livebackup. > > Thanks, > Jagane > > On 5/15/2011 7:11 AM, Stefan Hajnoczi wrote: >> >> On Sun, May 15, 2011 at 1:16 PM, Fred van Zwieten<fvzwie...@gmail.com> >> wrote: >>> >>> Background: >>> NetApp Snashot functionality gives you application consistent >>> snapshots of data. Just inform the app a snapshot is about to be made, >>> depending on the app, it needs to go in to some sort of backup mode, >>> of just stop and flush outstanding I/O. Next, a snapshot is made and >>> everything just runs on. Because of the underlying WAFL filesystem >>> design, the snapshot always points to the blocks at the time of the >>> creation without needing to do any COW. >>> >>> Layered on top op this functionality is SnapMirror, where the delta >>> between this snapshot and a previous snapshot (both being static in >>> nature), is asynchronously replicated to a second system. There, this >>> delta is applied to a copy of the disk as a local snapshot. >>> >>> This setup gives you application consistent data disks on a remote >>> location as a form of disaster tolerancy. The RPO is the >>> snapshot/snapmirror frequency. >>> >>> KVM: >>> My question is rather simple. Could something like this be implemented >>> with kvm-img and rsync and/or lvm? I've played with the backing_file >>> option, but this means I have to shutdown the vm a boot is on the new >>> file to let this work. >> >> This recent thread might interest you: >> http://lists.gnu.org/archive/html/qemu-devel/2011-05/msg00733.html >> >> Basically you have to cobble it together yourself at the moment but >> there is activity around live snapshots and dirty block tracking, so >> hopefully they will be available as APIs in the future. >> >> Stefan > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
