[josefbacik]

We have to look up the physical extent in the extent reference tree, so the cost is independent of the number of snapshots and more a function of the fragmentation of the extent tree. The metadata is all cached of course, but fragmentation means you are likely to not find the entries in cache.

The other aspect that I haven't talked about is our fsync performance is kind of shit compared to other fs'es. Now this does get better in the nocow case but it's still pretty heavy and needs optimization.