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

[Anthony Liguori <aliguori@us.ibm.com>]

On 07/05/2012 08:53 AM, Michael S. Tsirkin wrote:
> 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.

We have demonstrated better results at much higher IOP rates with virtio-blk in 
userspace so while these results are nice, there's no reason to believe we can't 
do this in userspace.

>> 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.

Users want something that has lots of features and performs really, really well. 
  They want everything.

Having one device type that is "fast" but has no features and another that is 
"not fast" but has a lot of features forces the user to make a bad choice.  No 
one wins in the end.

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.

> To me, it does not make sense to reject code just because in theory
> someone could write even better code.

There is no theory.  We have proof points with virtio-blk.

> Code walks. Time to marker matters too.

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.  We can demonstrate this with virtio-blk.  This isn't a 
theoretical argument.

> 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?

Asking a user to write a helper script is pretty awful...

>
>> 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?

It's an entirely different emulation path.  It's not a simple packet protocol 
like virtio-net.  It's a complex command protocol where the backend maintains a 
very large amount of state.

> Userspace virtio-scsi
> is pretty flexible - can't it supply matching inquiry/vpd/mode data
> so that switching is transparent to the guest?

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.

Regards,

Anthony Liguori

>
>> 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
>