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

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Message-ID: <500527BA.9000001@redhat.com>
Date:	Tue, 17 Jul 2012 10:52:10 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Asias He <asias@...hat.com>
CC:	Stefan Hajnoczi <stefanha@...il.com>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, kvm@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	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 10:29, Asias He ha scritto:
> So, vhost-blk at least saves ~6 syscalls for us in each request. 

Are they really 6?  If I/O is coalesced by a factor of 3, for example
(i.e. each exit processes 3 requests), it's really 2 syscalls per request.

Also, is there anything we can improve?  Perhaps we can modify epoll and
ask it to clear the eventfd for us (would save 2 reads)?  Or
io_getevents (would save 1)?

> I guess you mean qemu here. Yes, in theory, qemu's block layer can be
> improved to achieve similar performance as vhost-blk or kvm tool's
> userspace virito-blk has. But I think it makes no sense to prevent one
> solution becase there is another in theory solution called: we can do
> similar in qemu.

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.

So it begs the question, is it going to be used in production, or just a
useful reference tool?

Paolo
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