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Re: [Xen-devel] xen 4.1.1 + 3.0.0-rc5 dom0 + blktap2

To: Madjik <madjik@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] xen 4.1.1 + 3.0.0-rc5 dom0 + blktap2
From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date: Thu, 30 Jun 2011 11:30:08 +0100
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On Wed, 2011-06-29 at 03:06 +0100, Madjik wrote:
> Hi,
> I'm currently trying xen 4.1.1 with the 3.0.0-rc5 (heard that it now 
> contains the xen code to run as front or back xen domain).
> Everything seems to run fine, except that I am trying since a  few hours 
> to mount a .vhd file in the dom0.

Everything needed to run a guest is upstream but not everything needed
to run any guest, if you see what I mean. In particular the upstream
kernel contains blkback but not blktap so using a vhd would not be
expected to work today.

I'm afraid blktap is unlikely to be accepted upstream as it is. There is
a plan to remove the kernel component and have guests communicate
directly with the tapdisk process in userspace (via /dev/xen/gtndev et
al). There is no real downside to this since everything goes through
tapdisk anyway. I don't know how advanced that plan is currently.

With the 4.1 release you might be able to use the qdisk (qemu based
backend) to run a VHD but it is known not to perform all that well (and
I don't know off-hand how you enable it). Performance is, AIUI, better
with the upstream qemu port.


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