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

From: Stefan Hajnoczi
Date: Tue Jul 17 2012 - 07:36:32 EST

On Tue, Jul 17, 2012 at 9:29 AM, Asias He <asias@xxxxxxxxxx> wrote:
> On 07/16/2012 07:58 PM, Stefan Hajnoczi wrote:
>> Does the vhost-blk implementation do anything fundamentally different
>> from userspace? Where is the overhead that userspace virtio-blk has?
> Currently, no. But we could play with bio directly in vhost-blk as Christoph
> suggested which could make the IO path from guest to host's real storage
> even shorter in vhost-blk.

Wait :). My point is that writing new code without systematically
investigating performance means that we're essentially throwing random
things and seeing what sticks.

Adding bio mode would make vhost-blk and kvmtool more different.
It'll probably make vhost-blk slightly faster but harder to compare
against kvmtool. It's easier to start profiling before making that

The reason I said "special-purpose kernel module" is because kvmtool
could be suffering from a bottleneck that can be fixed. Other
userspace applications would also benefit from that fix - it would be
generally useful. Adding a vhost-blk kernel module works around this
but only benefits KVM specifically.

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