I was wondering if there was anyone who is trying to use the COW filesystem in a normal linux kernel?  Is this possible, or is it tied exclusively to the rest of the UML patch?  How difficult would it be to make a COW only patch?  I have a certain project that I'm working on that the COW filesystem (or really, any other "union" filesystem, but I can't find any that are actively developed or fully cooked) would be perfect for.  I have a read-only filesystem that, occasionally needs to temoroarily have some changed made to it, but then the need to be undone.  I've looked into various ways of doing this from "snapshotting," to user-controlled transactions on a journalling filesystem (no such thing seems to exist for ext3 or reiserfs) where I could explicitly begin a transaction, then roll back the transaction when I'm done (kinda like a database txn), etc.  The COW filesystem seems like it would be a perfect fit, but before I go trying to merge things into a non-UML kernel on my own I just want to see if anyone else has already done this or not.  Is anyone else even interested in such a thing?