On Fri, Mar 27, 2015 at 03:35:02PM +0000, Richard W.M. Jones wrote:
On Fri, Mar 27, 2015 at 03:21:25PM +0000, Stefan Hajnoczi wrote:
On Fri, Mar 27, 2015 at 12:31:41PM +0000, Richard W.M. Jones wrote:
On Fri, Mar 27, 2015 at 12:15:34PM +0000, Stefan Hajnoczi wrote:
On Thu, Mar 26, 2015 at 12:02:27AM +0100, Kashyap Chamarthy wrote:
So, something like?
. . . { 'execute': 'drive-backup', 'arguments': { 'device': 'drive-virtio-disk1', 'sync': 'full', 'target': 'nbd://localhost', 'mode': 'absolute-paths', 'format': 'qcow2' } . . .
Same question as yours, what is the NBD server going to run?
My only NBD testing so far has been with w/ NBD over Unix socket or over TCP[**].
For 'sync': 'full' mode qemu-nbd or nbd-server can be used as the server. You probably don't want 'format': 'qcow2', just raw data over NBD. That way the NBD server can implement whatever storage backend it likes (raw, qcow2, something else).
For incremental backup the NBD server would either be a qemu-nbd instance with a qcow2 image and backing file:
full-backup.img <- incremental-1.qcow2 <- incremental-2.qcow2
Or the NBD server would be a custom backup application that does something smart with the incoming NBD WRITE requests.
What I care about is connecting libguestfs to qemu and reading a snapshot at some point in time, even though the guest is still writing away to its disks. Is this possible with drive-backup (or otherwise)?
Yes, that is what drive-backup does.
New writes coming from the guest are held up until the old data has been written to the NBD target.
That way you get a point-in-time snapshot while the guest continues running.
I understand how that can work for backups, where you want to copy a whole disk consistently.
But libguestfs doesn't want to do a backup, nor get a copy of the whole disk, it just wants to access a scattering of blocks (maybe a few hundred) but at a single point in time, in as lightweight a manner as possible.
There is a way to do that. Create a temporary qcow2 backed by the disk: guest.img <- tmp.qcow2 Now use drive-backup with sync mode 'none' so only new guest writes cause old data to be stashed in tmp.qcow2 before it is overwritten. Finally, use the run-time NBD server for random access to tmp.qcow2. When you are done, kill the drive-backup job and delete tmp.qcow2. Stefan