The config.placementHistory collection implements a log of any change affecting the placement of every sharded nss in the cluster (each event is recorded as a separate document); it may therefore grow indefinitely if not periodically pruned.

On the other hand,

• a document storing placement information at time T is only useful if each involved shard hasn't pruned any entry with opTime >= T from its oplog
• such document can be safely removed if config.placementHistory contains at least one more recent entry referencing the same namespace

Based on such condition, the config server should implement a new mechanism to periodically clean up the content of config.placementHistory