Is there a way to create a copy-on-write copy of a directory?

I'm thinking of a situation where I would have something that creates a copy of a directory, tweaks a few files, and then does some processing on the result. This wold be done fairly often, maybe a few dozen times a day. (The exact use case is testing patch submissions; dupe the code, patch it, build/test/report/etc.)

What I'm looking for could be done by creating a new directory structure and populating it with hard links from the origonal. However this only works if all the tools you use delete and recreate files rather than edit them in place.

Is there a way to have the file system do copy-on-write for a file?

Note: I'm aware that many FSs use COW at a block level (all updates are done via writes to new blocks) but this is not what I want.

If you have your choice of platform for your fileserver, I'd go with a recent OpenSolaris build and use the deduplication feature of ZFS. This way copies of files would take up no additional space, and even common segments between files would not be replicated.

You can read all about ZFS deduplication at this post on Jeff Bonwick's Blog.