[versteegen]

How about if you have a NOCOW subvolume with just a few files on it, e.g. 3 VM images. Is it still expensive to check for snapshots? Does the cost of checking disappear if you have no snapshots of the volume? As ryao mentioned, you'd still get fragmentation if you have snapshots, so in that case is using NOCOW for a VM image counterproductive?

Seems like the expense of checking could be largely removed with clever enough metadata caching, which probably noone has had time to implement.

[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.