Re: Writing a tool for Shared Persistent Windows Boot Image

[Anthony Liguori <anthony@codemonkey.ws>]

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