The bookies do not clean up ledgers on their disk which exist in zookeeper but are not assigned to them by the ensemble definition. This happens if a bookie has a ledger, went offline, it was replicated elsewhere, and then the bookie comes back up. Then we have an extra copy of the same ledger.
Bookie should handle this case in the garbage collector. Since we will have to read the ledger metadata and go through its ensemble set to determine if the bookie exists in the ensemble, this is an expensive operation. Thus, we will only run this task once every day.