Hi, VirtIO SCSI [0] has been supported a while now by Linux and all kernels, but inside CloudStack we are not using it. There is a issue for this [1].
It would bring more (theoretical) performance to VMs, but one of the motivators (for me) is that we can support TRIM/DISCARD [2]. This would allow for RBD images on Ceph to shrink, but it can also give back free space on QCOW2 images if quests run fstrim. Something all modern distributions all do weekly in a CRON. Now, it is simple to swap VirtIO for VirtIO SCSI. This would however mean that disks inside VMs are then called /dev/sdX instead of /dev/vdX. For GRUB and such this is no problems. This usually work on UUIDs and/or labels, but for static mounts on /dev/vdb1 for example things break. We currently don't have any configuration method on how we want to present a disk to a guest, so when attaching a volume we can't say that we want to use a different driver. If we think that a Operating System supports VirtIO we use that driver in KVM. Any suggestion on how to add VirtIO SCSI support? Wido [0]: http://wiki.qemu.org/Features/VirtioSCSI [1]: https://issues.apache.org/jira/browse/CLOUDSTACK-8239 [2]: https://issues.apache.org/jira/browse/CLOUDSTACK-8104
