Re: [Qemu-devel] [patch 3/4] block stream: add support for partial streaming

[Stefan Hajnoczi]

On Mon, Jan 9, 2012 at 10:58 AM, Kevin Wolf <address@hidden> wrote:
> Am 04.01.2012 23:40, schrieb Stefan Hajnoczi:
>> Kevin: I think we need something like qcow2_snapshot_load_tmp() but it
>> returns a full new BlockDriverState.  The hard thing is that duping a
>> read-only snapshot qcow2 state leads to sharing and lifecycle problems
>> - what if we want to close the original BlockDriverState, will the
>> read-only snapshot state prevent this?
>
> Yes, creating new read-only BlockDriverStates from one image is exactly
> the thought that I had when reading this thread. The problem is that the
> BlockDriverStates wouldn't be fully independent. What if you delete the
> snapshot that is used by another BlockDriverState etc.?
>
> I think the least you would need is to have a separation between one
> BlockImage (which is a whole qcow2 file) and multiple BlockDriverStates
> (which is the backend that devices/NBD servers/whatever use). Not sure
> if such a fundamental block layer change is worth the effort.

If we want internal snapshots to be useful then I think this change is
necessary.

One thing that bothers me about an in-process NBD server is that
client access to data is tied to the qemu process lifetime.  From the
libvirt level this would mean being aware that the in-process NBD
server should be used when the VM is running and using qemu-nbd
instead when the VM is not running.  Transitions between the running
and not running state would not be very smooth unless it did something
crazy like an NBD proxy that multiplexes between the in-process and
the qemu-nbd servers.

Stefan