MMAP is a pluggable storage engine that was named after the mmap() Linux command. It maps files to the virtual memory and optimizes read calls. If you have a large file but needs to read just a small part of it, mmap() is much faster then a read() call that would bring the entire file to the memory.

One disadvantage is that you can't have two write calls being processed in parallel for the same collection. So, MMAP has collection-level locking (and not document-level locking as WiredTiger offers). This collection-locking is necessary because one MMAP index can reference multiples documents and if those docs could be updated simultaneously, the index would be inconsistent.