Re: [Xen-devel] Re: Writing a tool for Shared Persistent Windows Boot Image

["Jim Burnes" <jvburnes@xxxxxxxxx>]

Dan,

Since blktap looks broken on HVM for the forseeable future (a real shame if you ask me), what's the next best thing? LVM snapshots?  How can I get LVM snapshots to use use less main memory?  Will it help if I decrease the max size of the copy-on-write snapshot?  Usually our copy-on-write area is no larger than 20MB.

Jim Burnes

On 6/28/07, Daniel P. Berrange <berrange@xxxxxxxxxx> wrote:

On Thu, Jun 28, 2007 at 12:18:47PM -0600, Jim Burnes wrote:
> Anthony (and anyone else listening),
>
> I've installed Centos 5, Xen 3.1 (from the Xensource RPMs) and installed a
> base Windows image to an LVM to test.
>
> The installation wen't fairly well and I resolved various issues like
> getting the VMs to render to VNC displays properly.
>
> I even used LVM snapshotting to do some quick copy-on-write tests.  In that
> configuration I've had up to 7 Windows VMs running at the same time, on
> separate IPs, writing to LVM copy-on-write snapshots.  The only problem is
> that LVM cow snapshots consume too much main memory.
>
> So I proceeded to my final test which was to replace the LVM storage with
> QCOW storage and see how many Windows VMs I could run concurrently.  (I'm
> trying for 12).
>
> The only cow tool I had was qcow-create, so I downloaded the qemu RPMs and
> installed them.
>
> I used img2qcow to convert my installed Windows LVM image to the qcow base
> image.
> I used qcow-img to create a copy-on-write snapshot of the base image.
> I tried to boot with this as the new storage type by specifiying:
>
> disk = [ 'tap:qcow:/var/lib/xen/image/winflp.img,hda1,w',
> 'file:/var/lib/xen/install/winflp.iso,hdc:cdrom,r']
>
> (and other variations, by using 'hda' instead of 'hda1', using 'tap:aio'
> instead of 'tap:qcow')
>
> But everytime I try to start the vm by 'xm create' it seems to create the VM
> and the VM immediately exits, leaving hardly any trace of even booting.  I
> tried booting directly to hard drive C: and I tried booting to the CDROM.
> Booting to the CDROM worked fine but reported no attached hard drive.
>
> The only thing I see is a trace in the xend.log file which indicates that
> xend (or the VM) is attempting to generate some sort of hotplug event for
> tap.  Then the whole VM shuts down.
>
> Is there something I need to do before tap:qcow works?
>
> I suspect, but am having a hard time proving that either:
>
> (1) My version of Xen 3.1 doesn't support tap:qcow or

It is not possible to use  blktap with HVM guests - there was code which tried
to make it work, but its utterly broken[1]. In my spare time I'm trying to fix
it, but no ETA.

Dan.
[1] it is calling APIS in XenD which no longer exist & has a try..except
    block which silently catches & ignores the errors, so you never notice
    that it failed.
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