I really like btrfs for my backup machine, but it sometimes manages to hang when it's cleaning up deleted snapshots. This is a problem that's much worse when on a hard drive or fragmented file system, but I've gotten it to happen even on a recently-balanced drive with a small amount of snapshots.

I recently had a server be crippled by running snapper on default settings for a few months. And after a couple days of balancing (which desperately needs a throttle control) it wasn't much better, so I gave up on having it run btrfs.

I think the record I managed was slightly over two minutes of btrfs blocking all disk I/O. Something is deeply wrong with how it organizes transactions.