The parallelCollectionScan command does not work over mongos.

This is mentioned in passing on the Java driver Getting Started page (http://docs.mongodb.org/ecosystem/tutorial/getting-started-with-java-driver/#parallelscan), specifically here:

ParallelScan does not work via mongos.

It is also mentioned on Google Groups here:

https://groups.google.com/forum/#!topic/mongodb-user/Ss-uhnUbUbU

However, this limitation is not documented on the page for the parallelCollectionScan command itself (http://docs.mongodb.org/manual/reference/command/parallelCollectionScan/)