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

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/04/2012 11:40 PM, Stefan Hajnoczi wrote:
> What you want sounds almost like an NBD server that can be
> launched/stopped while qemu is already running a VM.  This could be a
> QEMU monitor command like:
> nbd-start tcp::1234 virtio-disk0 --snapshot 20120104
>
> It would be possible to stop the server using the same<socket, drive,
> snapshot>  tuple.  Note the server needs to provide read-only access,
> allowing writes probably has little use and people will hose their
> data.

That makes sense, just like most qemu-img commands have an equivalent in 
the monitor for online usage.

> Paolo: I haven't looked at the new and improved NBD server yet.  Does
> this sound doable?

Yes, indeed.  It should not be hard.  The NBD server is now entirely 
asynchronous, and by using the main loop the integration is very much 
simplified.

Briefly, nbd.c now has a simple server API:

     typedef struct NBDExport NBDExport;
     typedef struct NBDClient NBDClient;

     NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset,
                               off_t size, uint32_t nbdflags);
     void nbd_export_close(NBDExport *exp);
     NBDClient *nbd_client_new(NBDExport *exp, int csock,
                               void (*close)(NBDClient *));

... that takes care of everything except creating the server socket and 
accepting clients from it.  Which is actually even better, because 
instead of having a generic NBD server you could start one on a file 
descriptor that you pass via SCM_RIGHTS (aka getfd).

> 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?

We can prevent closing the parent BDS until all its children are gone.

Paolo