The compact command is per collection, does not reclaim space on the file system, and blocks use of the database The repairDatabase is per database, does reclaim space, and blocks use of the server. Neither run automatically, and therefore must be scheduled externally.

This is a request for compaction of an entire database that can be done without blocking use of the server or even the database, similar to the VACUUM command in Postgres. Note that compact/repairDatabase is roughly equivalent to a VACUUM FULL.