Re: [libvirt] [RFC v2] external (pull) backup API

Hi, all. This is another RFC on pull backup API. This API provides means to read domain disks in a snapshotted state so that client can back them up as well as means to write domain disks to revert them to backed up state. The previous version of RFC is [1]. I'll also describe the API implementation details to shed light on misc qemu dirty bitmap commands usage. This API does not use existent disks snapshots. Instead it introduces snapshots provided by qemu's blockdev-backup command. The reason is we need snapshotted disk state only temporarily for duration of backup operation and newly introduced snapshots can be easily discarded at the end of operation without block commit operation. Technically difference is next. On usual snapshot we create new image backed by original and all new data goes to the new image thus original image stays in a snapshotted state. In temporary snapshots we create new image backed by original and all new data still goes to the original image but before new data is written old data to be overwritten is popped out to the new image thus we get snapshotted state thru new image. Disks snapshots as well as disks itself are avaiable to read/write thru qemu NBD server.