I can confirm that btrfs has major problems with tons of snapshots. The entire system will hang for minutes on random intervals and go completely unresponsive for hours when using admin commands. The many snapshots use case is pretty common when dealing with containers, even it everything works fine for simple setups.
How many is tons? I've not had such problems with hundreds of snapshots. There are reports on the btrfs list with problems involving many thousands. I have no idea how ZFS snapshots perform when there are hundreds to thousands.
The other thing that matters a ton is kernel version, the development is so heavy there are thousands of insertions+deletions per kernel cycle that even in a year it's ordinary to consider those kernels "old" on the list. Most distros by default use old kernels by Btrfs development standards. The exception is Fedora, and openSUSE who do lots of backports explicitly to support Btrfs.
Yeah, I'm talking 10k+, so this is only specific types of applications. In basic benchmarks, ZFS seemed to perform fine well into with 10s of thousands of snapshots. This is still far more snapshots than basic containers will create, so it's really only a problem for situations where you're doing tons of checkpointing.
Yeah I've been using Docker on Btrfs and haven't had issues. I do wonder why it takes so long (maybe 4 seconds) to delete containers, when deleting the snapshot for that container is instant. I haven't found dm based containers to be any faster.
