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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/07/2012 15:53, Michael S. Tsirkin ha scritto:
> 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.

It's not about writing better code.  It's about having two completely
separate SCSI/block layers with completely different feature sets.

> 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?

We could add hooks for vhost-scsi in the SCSI devices and let them
configure themselves.  I'm not sure it is a good idea.

>> and doesn't support -device;
> 
> This needs to be fixed I think.

To be clear, it supports -device for the virtio-scsi HBA itself; it
doesn't support using -drive/-device to set up the disks hanging off it.

>> 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?

It cannot support anyway the whole feature set unless you want to port
thousands of lines from the kernel to QEMU (well, perhaps we'll get
there but it's far.  And dually, the in-kernel target of course does not
support qcow2 and friends though perhaps you could imagine some hack
based on NBD.

Paolo