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.
