It sounds rather like VM forking, except you want to be able to can the base image for later re-instantiation. I guess you would create the in-memory read-only base VM on demand from the canned image when the first CoW VM is created, and garbage-collect it when the last CoW VM is destroyed.

The idea of taking a small basic set of VM images and customising their configuration when instantiating them is sane. For the memory sharing we would like to investigate a more general mechanism (e.g., shared buffer cache indexed by content hash) which would optimise memory usage not just in your scenario but also in a whole bunch of others.

-- Keir