Re: [PATCH 0/5] Add vhost-blk support

From: Michael S. Tsirkin
Date: Tue Jul 17 2012 - 06:48:51 EST

On Tue, Jul 17, 2012 at 12:14:33PM +0200, Paolo Bonzini wrote:
> Il 17/07/2012 11:45, Michael S. Tsirkin ha scritto:
> >> So it begs the question, is it going to be used in production, or just a
> >> useful reference tool?
> >
> > Sticking to raw already makes virtio-blk faster, doesn't it?
> > In that vhost-blk looks to me like just another optimization option.
> > Ideally I think user just should not care where do we handle virtio:
> > in-kernel or in userspace. One can imagine it being enabled/disabled
> > automatically if none of the features unsupported by it are used.
> Ok, that would make more sense. One difference between vhost-blk and
> vhost-net is that for vhost-blk there are also management actions that
> would trigger the switch, for example a live snapshot.
> So a prerequisite for vhost-blk would be that it is possible to disable
> it on the fly while the VM is running, as soon as all in-flight I/O is
> completed.

It applies for vhost-net too. For example if you bring link down,
we switch to userspace. So vhost-net supports this switch on the fly.

> (Note that, however, this is not possible for vhost-scsi,
> because it
> really exposes different hardware to the guest. It must not happen that
> a kernel upgrade or downgrade toggles between userspace SCSI and
> vhost-scsi, for example).

I would say this is not a prerequisite for merging in qemu.
It might be a required feature for production but it
is also solvable at the management level.

Imagine an "enable-live-snapshots" flag in libvirt, on by default.
Can only be changed while guest is down. If you turn it off,
you get a bit more speed since vhost-blk/vhost-scsi gets enabled.
Also pls note that a backend can support live snapshots.
If it does libvirt thinkably could detect that
and enable vhost-scsi even with enable-live-snapshots on.

> >> having to
> >> support the API; having to handle transition from one more thing when
> >> something better comes out.
> >
> > Well this is true for any code. If the limited featureset which
> > vhost-blk can accelerate is something many people use, then accelerating
> > by 5-15% might outweight support costs.
> It is definitely what people use if they are interested in performance.
> Paolo

In that case it seems to me we should stop using the featureset as
an argument and focus on whether the extra code is worth the 5-15% gain.
No one seems to have commented on that so everyone on list thinks that
aspect is OK? Any explicit ACKs?

Kernel merge windows is coming up and I would like to see whether
any of vhost-blk / vhost-scsi is going to be actually used by userspace.
I guess we could tag it for staging but would be nice to avoid that.

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