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

To: Jim Burnes <jvburnes@xxxxxxxxx>
Subject: [Xen-devel] Re: Writing a tool for Shared Persistent Windows Boot Image
From: Anthony Liguori <anthony@xxxxxxxxxxxxx>
Date: Thu, 21 Jun 2007 13:11:28 -0500
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Jim Burnes wrote:
Before, in my "Hard Problem" email I was trying to communicate a design issue were trying to solve with Xen.

This is what we need to do:

1) Deploy 24 Windows XP VMs in parallel.

I have strong doubts that this would be kosher from a licensing perspective, however...

2) Boot them from a shared Windows XP C: drive.
3) Since this is a read-only shared image we obviously can't have multiple VM's writing to it. 4) All writes to the boot image for logging, registry and other purposes should be diverted to an auxiliary shadow drive specific to each VM.

If you start with a single image, and then create "COW" files using the qcow format, then you can have a shared base image.

5) After we shut down the VM we need to mount and examine the contents of the shadow drive

Mounting is tricky. If you look on qemu-devel, you'll find a couple references to tool that allow you to mount a qcow file (usually with nbd).

6) When we are done examining the contents of the shadow drive, we need to fast format it for the next VM to use.

You can just delete the cow file and create a new one.

Regards,

Anthony Liguori

Is this supported natively in Xen? What does everyone else who needs to run a lot of Windows VMs do? There must be a way to support shared images.

The reason I posted this to xen-devel is that I could probably implement a UnionFS for Windows by writing a kernel hook and intercepting all reads and writes to the C: drive, but I don't have enough time to do that right now. Because of schedule constraints, if we don't find a way to do this in Xen/XenSource we'll have to drop Xen and move on to VMWare ESX at considerable cost to our project.

Are there any senior Xen software engineers out there who've done this or who might know how?

Thanks,

Jim Burnes


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