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 tell LevelDB how much memory it can use up by setting leveldb.maximum_memory.percent.
Configuring LevelDB memory:
LevelDB means business with its allocated memory. It will eventually end up with the configured max, making it look like there's a memory leak, or even triggering OOM kills. Keep that in mind when configuring the percentage of RAM you give to LevelDB. Historically, the configured default was at 70% percent of RAM, which is too high for a lot of use cases and can be safely lowered.
(e)LevelDB exposes a couple of additional configuration values that we link here for the sake of completeness. You can change all the values mentioned in the eleveldb schema file. VerneMQ mostly uses the configured defaults, and for most use cases it should not be necessary to change those.