linux-kernel - Re: [Xen-devel] Re: [PATCH v3] xen block backend.

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Message-ID: <1303373003.5997.337.camel@zakaz.uk.xensource.com>
Date:	Thu, 21 Apr 2011 09:03:23 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"konrad@...nel.org" <konrad@...nel.org>
Subject: Re: [Xen-devel] Re: [PATCH v3] xen block backend.

On Thu, 2011-04-21 at 08:28 +0100, Ian Campbell wrote:
> By contrast blktap has a userspace component so it's not all that
> surprising that it turns out to be roughly equivalent to qdisk. (bear in
> mind that Stefano's tests were very rough and ready initial tests, not
> that anyone expects a more thorough benchmarking treatment to really
> change the result). Nobody I know of thinks blktap should go upstream
> since as you say there is no reason not to punt the kernel side part
> into userspace too.

BTW about the only nice property blktap has as it currently stands over
this plan is that it exports an actual block device from vhd, qcow etc
files (in some sense blktap is a loopback driver for complex disk image
file formats). It turns out to occasionally be quite useful to be able
to mount such files, even on non-virtualisation systems (in its current
incarnation blktap has no dependency on Xen).

Having removed the kernel component (or switched to qdisk) we will
probably end up running a blkfront to provide such block devices (sadly
Xen dependent) or, more likely, putting something like an NBD server
into the userspace process.

Ian.

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