The biggest cost to copy-on-write data structures is write magnification. Adding a log to amortize that cost helps a lot. That's what the ZFS intent log does.

The biggest cost to copy-on-write data structures is write magnification. Adding a log to amortize that cost helps a lot. That's what the ZFS intent log does. Any CoW b-tree scheme can benefit from that approach. The benefits of CoW data structures are tremendous: - easy to reason about - easy to multi-thread for reading - O(1) snashopts (and clones) The downsides of CoW data structures are mainly: - the need to amortize write magnification - difficulty in multi-threading writes