VerneMQ uses Google's LevelDB as a fast storage backend for messages and subscriber information. Each VerneMQ node runs its own embedded LevelDB store.
There's not much you need to know about LevelDB and VerneMQ. One really important thing to note is that LevelDB manages its own memory. This means that VerneMQ will not allocate and free memory for LevelDB. Instead you'll have to configure a configuration value in vernemq.conf that tells LevelDB how much memory it can use up.
Configuring LevelDB memory:
leveldb.maximum_memory.percent = 20