In 2.4 and prior, renameCollection across databases would not preserve autoIndexId property of the collection.

Starting with 2.5.5 it does respect the autoIndexId setting and preserves it in the new collection.

Since MR uses a temp DB and then renames the temp collection into the target DB, and MR uses autoIndexId "false" this breaks replication performance when this causes MR results to have on "_id" index in the result collection.