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

To: Jim Burnes <jvburnes@xxxxxxxxx>, Xen Mailing List <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: Writing a tool for Shared Persistent Windows Boot Image
From: Anthony Liguori <anthony@xxxxxxxxxxxxx>
Date: Thu, 21 Jun 2007 15:39:59 -0500
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Jim Burnes wrote:
If you start with a single image, and then create "COW" files using the qcow format, then you can have a shared base image.


Let me make sure I understand your answer. COW files are sparse storage for the QEMU environment (which tools you use for Xen).

We want a static filesystem image that represents a snapshot of a Windows XP system right after boot. When we activate that image we want to perform a few housekeeping issues (like set the MAC and re-DHCP etc), but we also want to make sure that any writes to the image are redirected to an overlay writable file system. In other words we want a single shared image of the OS itself with all writes going to the COW / shadow image of that specific VM.

COW == Copy On Write. It's a separate file that only stores the data that has been written since the cow was created.

Is the COW file you speak of overlayed on top of a single static Windows image or does the COW file contain the entire Windows XP boot image plus any writes.

It just contains the writes after the COW was created.

I know this depends on your previous answer, but if we delete the COW doesn't that delete the XP boot image also?

If you delete the COW, the underlying base image is unaffected.

If the COW file is used as an extent-space to the static Windows XP image, then that should work.

Otherwise I think it would require a recreation of the full Windows XP image. Ideally that image can be kept static as the continual deletion and re-creation of relatively large COW files would be time-consuming and would tend to fragment hard disk space.

You really shouldn't worry about disk fragmentation but that's a whole other thread :-)

Regards,

Anthony Liguori

Other than that, I'm very grateful for your kind assistance.

Thanks again,

Jim Burnes
Boulder, CO




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