RE: http://docs.mongodb.org/manual/reference/command/compact

The implementation of the "compact" command assumes a set of MMAP-based limitations, such as taking an exclusive database lock and, in the case of a secondary, force in to RECOVERING mode - neither of these actions is essential when running WiredTiger.

Additionally, WiredTiger can release free space back to the storage system without needing to rebuild everything, unlike MMAP, but this ability is not exploited.

Access to these various extra functionality of WiredTiger specifically in regards to compaction is the purpose of this ticket. What form this takes is left for discussion.