It would be helpful for customers to just add a link or note about our recommendation to use the XFS file system with WiredTiger in the notes about how to "convert" from MMAPV1 to WiredTiger:

https://docs.mongodb.org/manual/release-notes/3.0-upgrade/#change-storage-engine-to-wiredtiger
perhaps a link to here: https://docs.mongodb.org/manual/administration/production-notes/#kernel-and-file-systems

I am working with a customer now who converted but did not change filesystems, so it's a bunch of extra work now to get things correct.