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

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Date:	Tue, 17 Jul 2012 12:14:33 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Asias He <asias@...hat.com>, Stefan Hajnoczi <stefanha@...il.com>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Add vhost-blk support

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.

(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).

>> 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
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