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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
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