To use LevelDB, follow these steps:

1) Add the following to your rippled.cfg file:
[node_db]
LevelDB

2) If you have no databases, you can just start rippled. If you have
databases you don't care about, just remove them. Otherwise, start rippled
with '--import' to import your existing hashnode database into LevelDB.
After you import the hashnode database, either delete (or move) the
'hashnode.db', 'hashnode.db-shm' and 'hashnode.db-wal' files from your
database directory. Do not touch the LevelDB files in the 'hashnode'
directory.