On Thu, Jul 05, 2012 at 04:47:43PM +0200, Paolo Bonzini wrote: > Il 05/07/2012 16:40, Michael S. Tsirkin ha scritto: > >> virtio-scsi is brand new. It's not as if we've had any significant > >> time to make virtio-scsi-qemu faster. In fact, tcm_vhost existed > >> before virtio-scsi-qemu did if I understand correctly. > > Yes. > > > Can't same can be said about virtio scsi - it seems to be > > slower so we force a bad choice between blk and scsi at the user? > > virtio-scsi supports multiple devices per PCI slot (or even function), > can talk to tapes, has better passthrough support for disks, and does a > bunch of other things that virtio-blk by design doesn't do. This > applies to both tcm_vhost and virtio-scsi-qemu. > > So far, all that virtio-scsi vs. virtio-blk benchmarks say is that more > benchmarking is needed. Some people see it faster, some people see it > slower. In some sense, it's consistent with the expectation that the > two should roughly be the same. :) Anyway, all I was saying is new technology often lacks some features of the old one. We are not forcing new inferior one on anyone, so we can let it mature it tree. > >> But guest/user facing decisions cannot be easily unmade and making > >> the wrong technical choices because of premature concerns of "time > >> to market" just result in a long term mess. > >> > >> There is no technical reason why tcm_vhost is going to be faster > >> than doing it in userspace. > > > > But doing what in userspace exactly? > > Processing virtqueues in separate threads, switching the block and SCSI > layer to fine-grained locking, adding some more fast paths. > > >> Basically, the issue is that the kernel has more complete SCSI > >> emulation that QEMU does right now. > >> > >> There are lots of ways to try to solve this--like try to reuse the > >> kernel code in userspace or just improving the userspace code. If > >> we were able to make the two paths identical, then I strongly > >> suspect there'd be no point in having tcm_vhost anyway. > > > > However, a question we should ask ourselves is whether this will happen > > in practice, and when. > > It's already happening, but it takes a substantial amount of preparatory > work before you can actually see results. > > Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
