From the FAQ: https://www.arangodb.com/faq/

"A database in ArangoDB can be larger than the available main memory. Although ArangoDB stores all data on disk for durability reasons, it is a “mostly main memory database”. This means that the working set (the set of pages that are frequently accessed) should fit into main memory. It’s left to the operating system to determine the working set and to transfer pages between main memory and secondary storage. The data that are currently not needed are kept only on secondary storage.

"This is in principle also true for indexes: a part of the index (the working set) could be in main memory and the remainder could be on secondary storage if not used frequently."

You may also be interested in the roadmap: https://www.arangodb.com/roadmap/