Virtio-scsi-for-bdm

Add virtio-scsi bus support for block device mapping
The corresponding BP is https://blueprints.launchpad.net/nova/+spec/add-virtio-scsi-bus-for-bdm

Virtio SCSI
VirtIO SCSI is a new para-virtualized SCSI controller device for KVM instances. It has been designed to replace virtio-blk, increase it's performance and improve scalability. The interface is capable of handling multiple block devices per virtual SCSI adapter, keeps the standard scsi device naming in the guests (e.x /dev/sda) and support SCSI devices passthrough.

Goals

 * 1) Boot from a cinder volume with virtio-scsi bus type

Use cases

 * 1) Improve performance in I/O-intensive applications

Libvirt Driver Support
When using a volume with "scsi" bus type and not specify a scsi controller explicitly in the domain xml, libvirt will automatically generate a "lsi" controller. Virtio-Scsi controller can be specified as follows:



Related works

 * 1) Block-device-mapping-v2 extension has already support attach or boot from a volume which is connected to Instance via scsi bus type and multiple disk types. But cannot generate a virtio-scsi controller.
 * 2) Libvirt-virtio-scsi-driver BP ([2]) whose milestone target is icehouse-3 is aim to support generate a virtio-scsi controller when using an image with virtio-scsi property. But it seems not to take boot-from-volume into account.

Implementation
API support

Add "virtio-scsi" bus type to block-device-mapping-v2 extension,  as follows: --block-device id=xxx,source=volume,dest=volume,bus=virtio-scsi,type=lun

Boot from Virtio-scsi Passthrough volume

In libvirt driver, append the virtio-scsi controller (will be introduced by libvirt-virtio-scsi-driver BP [1]) if we booting from a volume with "virtio-scsi" bus type.