Re: [PATCH v0 0/2] virtio-blk and vhost-user-blk cross-device migration

[Roman Kagan]

On Mon, Oct 04, 2021 at 11:11:00AM -0400, Michael S. Tsirkin wrote:
> On Mon, Oct 04, 2021 at 06:07:29PM +0300, Denis Plotnikov wrote:
> > It might be useful for the cases when a slow block layer should be replaced
> > with a more performant one on running VM without stopping, i.e. with very 
> > low
> > downtime comparable with the one on migration.
> > 
> > It's possible to achive that for two reasons:
> > 
> > 1.The VMStates of "virtio-blk" and "vhost-user-blk" are almost the same.
> >   They consist of the identical VMSTATE_VIRTIO_DEVICE and differs from
> >   each other in the values of migration service fields only.
> > 2.The device driver used in the guest is the same: virtio-blk
> > 
> > In the series cross-migration is achieved by adding a new type.
> > The new type uses virtio-blk VMState instead of vhost-user-blk specific
> > VMstate, also it implements migration save/load callbacks to be compatible
> > with migration stream produced by "virtio-blk" device.
> > 
> > Adding the new type instead of modifying the existing one is convenent.
> > It ease to differ the new virtio-blk-compatible vhost-user-blk
> > device from the existing non-compatible one using qemu machinery without any
> > other modifiactions. That gives all the variety of qemu device related
> > constraints out of box.
> 
> Hmm I'm not sure I understand. What is the advantage for the user?
> What if vhost-user-blk became an alias for vhost-user-virtio-blk?
> We could add some hacks to make it compatible for old machine types.

The point is that virtio-blk and vhost-user-blk are not
migration-compatible ATM.  OTOH they are the same device from the guest
POV so there's nothing fundamentally preventing the migration between
the two.  In particular, we see it as a means to switch between the
storage backend transports via live migration without disrupting the
guest.

Migration-wise virtio-blk and vhost-user-blk have in common

- the content of the VMState -- VMSTATE_VIRTIO_DEVICE

The two differ in

- the name and the version of the VMStateDescription

- virtio-blk has an extra migration section (via .save/.load callbacks
  on VirtioDeviceClass) containing requests in flight

It looks like to become migration-compatible with virtio-blk,
vhost-user-blk has to start using VMStateDescription of virtio-blk and
provide compatible .save/.load callbacks.  It isn't entirely obvious how
to make this machine-type-dependent, so we came up with a simpler idea
of defining a new device that shares most of the implementation with the
original vhost-user-blk except for the migration stuff.  We're certainly
open to suggestions on how to reconcile this under a single
vhost-user-blk device, as this would be more user-friendly indeed.

We considered using a class property for this and defining the
respective compat clause, but IIUC the class constructors (where .vmsd
and .save/.load are defined) are not supposed to depend on class
properties.

Thanks,
Roman.