During the Xen Summit at Oracle, Patrick Colp announced the release of the source code for his VM Snapshots project.  He writes:

I've been working on VM snapshots/CoW for Xen (as seen at Xen Summit this year). I'm happy to release my first version.

There are some known issues with capturing page dirties for HVM guests. I think it's related to the QEMU code (I believe I'm not catching the pages that QEMU dirties). However, it works with both 32-bit and 64-bit PV guests.

This release includes modifications to Linux, Xen, and some tools (a library, FUSE fs, and testing tool).

The FUSE file system will take a snapshot if you try to create a file in the directory you mount the xencowfs file system too.

The testing tool pauses a domain, enables CoW, takes a dump of the CoW image, takes two live memory dumps, unpauses the domain for a bit, then takes another CoW image. It compares all the images and reports on which
pages are different.

These patches are against xen-unstable revision 19425, however I had no trouble patching them against the current revision of xen-unstable (19553).

Please remember this is an alpha release, so there is likely to be some problems. Please let me know if you find any!

Thanks to Stephen Spector and Xen.org.