Bitcask can guarantee one disk seek, whereas LevelDB will do one disk seek per level, so at least from that perspective, it can't be better.
Level also has to look down the entire tree if a key is missing. This means inserts end up being more expensive than reads or updates (which are all just a hash lookup in Bitcask).
"Bitcask can guarantee one disk seek, whereas LevelDB will do one disk seek per level, so at least from that perspective, it can't be better."
Yep, this is a standard tradeoff. When you want your data to be iterable, you have to take the hit. In practice (I oversee a large cassandra cluster), this hit happens about ~1% of the time, which is either a lot, or a little, depending on your constraints.
"Level also has to look down the entire tree if a key is missing."
This is why Cassandra has a bloom filter on top of a very similar data store.
LevelDB is there as the replacement for those who are currently using Innostore as their backend and not for those who have a dataset that fits bitcask.
Level also has to look down the entire tree if a key is missing. This means inserts end up being more expensive than reads or updates (which are all just a hash lookup in Bitcask).