Re: [PATCH 0/6] tcm_vhost/virtio-scsi WIP code for-3.6

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 05, 2012 at 12:22:33PM +0200, Paolo Bonzini wrote:
> Il 05/07/2012 03:52, Nicholas A. Bellinger ha scritto:
> > 
> > fio randrw workload | virtio-scsi-raw | virtio-scsi+tcm_vhost | bare-metal raw block
> > ------------------------------------------------------------------------------------
> > 25 Write / 75 Read  |      ~15K       |         ~45K          |         ~70K
> > 75 Write / 25 Read  |      ~20K       |         ~55K          |         ~60K
> 
> This is impressive, but I think it's still not enough to justify the
> inclusion of tcm_vhost.  In my opinion, vhost-blk/vhost-scsi are mostly
> worthwhile as drivers for improvements to QEMU performance.  We want to
> add more fast paths to QEMU that let us move SCSI and virtio processing
> to separate threads, we have proof of concepts that this can be done,
> and we can use vhost-blk/vhost-scsi to find bottlenecks more effectively.

A general rant below:

OTOH if it works, and adds value, we really should consider including code.
To me, it does not make sense to reject code just because in theory
someone could write even better code. Code walks. Time to marker matters too.
Yes I realize more options increases support. But downstreams can make
their own decisions on whether to support some configurations:
add a configure option to disable it and that's enough.

> In fact, virtio-scsi-qemu and virtio-scsi-vhost are effectively two
> completely different devices that happen to speak the same SCSI
> transport.  Not only virtio-scsi-vhost must be configured outside QEMU

configuration outside QEMU is OK I think - real users use
management anyway. But maybe we can have helper scripts
like we have for tun?

> and doesn't support -device;

This needs to be fixed I think.

> it (obviously) presents different
> inquiry/vpd/mode data than virtio-scsi-qemu,

Why is this obvious and can't be fixed? Userspace virtio-scsi
is pretty flexible - can't it supply matching inquiry/vpd/mode data
so that switching is transparent to the guest?

> so that it is not possible to migrate one to the other.

Migration between different backend types does not seem all that useful.
The general rule is you need identical flags on both sides to allow
migration, and it is not clear how valuable it is to relax this
somewhat.

> I don't think vhost-scsi is particularly useful for virtualization,
> honestly.  However, if it is useful for development, testing or
> benchmarking of lio itself (does this make any sense? :)) that could be
> by itself a good reason to include it.
> 
> Paolo

-- 
MST