Description
Regarding this detail: http://docs.mongodb.org/manual/reference/command/compact/#blocking
During a compaction in a replica-set this suggests that operations would be blocked until the compaction completes, but this is often not the case.
The warning at the replica-sets section hints as to why: http://docs.mongodb.org/manual/reference/command/compact/#replica-sets
"On secondaries, the compact command forces the secondary to enter RECOVERING state."
Queries received by a node in other than SECONDARY or PRIMARY states fail with an error message, something to the effect of "not PRIMARY or SECONDARY". The ops do not block.