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

From: Michael S. Tsirkin
Date: Tue Jul 17 2012 - 05:51:15 EST

On Tue, Jul 17, 2012 at 11:32:45AM +0200, Paolo Bonzini wrote:
> Il 17/07/2012 11:21, Asias He ha scritto:
> >> It depends. Like vhost-scsi, vhost-blk has the problem of a crippled
> >> feature set: no support for block device formats, non-raw protocols,
> >> etc. This makes it different from vhost-net.
> >
> > Data-plane qemu also has this cripppled feature set problem, no?
> Yes, but that is just a proof of concept. We can implement a separate
> I/O thread within the QEMU block layer, and add fast paths that resemble
> data-path QEMU, without limiting the feature set.
> > Does user always choose to use block devices format like qcow2? What
> > if they prefer raw image or raw block device?
> If they do, the code should hit fast paths and be fast. But it should
> be automatic, without the need for extra knobs. aio=thread vs.
> aio=native is already one knob too much IMHO.

Well one extra knob at qemu level is harmless IMO since
the complexity can be handled by libvirt. For vhost-net
libvirt already enables vhost automatically dependeing on backend
used and I imagine a similar thing can happen here.

> >> So it begs the question, is it going to be used in production, or just a
> >> useful reference tool?
> >
> > This should be decided by user, I can not speak for them. What is wrong
> > with adding one option for user which they can decide?
> Having to explain the user about the relative benefits;

This can just be done automatically by libvirt.

> having to
> support the API; having to handle transition from one more thing when
> something better comes out.
> Paolo

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.

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