Re: [libvirt] [Qemu-devel] [RFC] live snapshot, live merge, live block migration

On Sun, May 22, 2011 at 10:52 AM, Dor Laor <dlaor(a)redhat.com&gt; wrote: > On 05/20/2011 03:19 PM, Stefan Hajnoczi wrote: >> >> I'm interested in what the API for snapshots would look like. >> Specifically how does user software do the following: >> 1. Create a snapshot >> 2. Delete a snapshot >> 3. List snapshots >> 4. Access data from a snapshot > > There are plenty of options there: >  - Run a (unrelated) VM and hotplug the snapshot as additional disk This is the backup appliance VM model and makes it possible to move the backup application to where the data is (or not, if you have a SAN and decide to spin up the appliance VM on another host).  This should be perfectly doable if snapshots are "volumes" at the libvirt level. A special-case of the backup appliance VM is using libguestfs to access the snapshot from the host.  This includes both block-level and file system-level access along with OS detection APIs that libguestfs provides. If snapshots are "volumes" at the libvirt level, then it is also possible to use virStorageVolDownload() to stream the entire snapshot through libvirt: http://libvirt.org/html/libvirt-libvirt.html#virStorageVolDownload Summarizing, here are three access methods that integrate with libvirt and cover many use cases: 1. Backup appliance VM.  Add a readonly snapshot volume to a backup appliance VM.  If shared storage (e.g. SAN) is available then the appliance can be run on any host.  Otherwise the appliance must run on the same host that the snapshot resides on. 2. Libguestfs client on host.  Launch libguestfs with the readonly snapshot volume.  The backup application runs directly on the host, it has both block and file system access to the snapshot. 3. Download the snapshot to a remote host for backup processing.  Use the virStorageVolDownload() API to download the snapshot onto a libvirt client machine.  Dirty block tracking is still useful here since the virStorageVolDownload() API supports <offset, length> arguments.