LMDB does ACID MVCC using copy-on-write with no garbage collection or compaction needed. It delivers consistent, deterministic write performance with no pauses. It is actually now in use in a number of soft realtime systems.
I was specifically thinking of LMDB as a counter-example when I wrote that it's not impossible, just hard to do well. A much more sensible set of tradeoffs than LevelDB.
