Concerning this configuration option:
http://docs.mongodb.org/master/reference/configuration-options/#storage.wiredTiger.collectionConfig.blockCompressor

1. The docs do not say what happens if this parameter is changed after start. We need some elaboration on the behavior of this setting.
2. The references to compressing indexes seem erroneous. It may be this setting affects indexes too (the two are inextricably linked at this time), but it primarily affects collection data.