ATM the MongoDB caches Store creates a database for each run instance, database name being an UUID in order to make sure that multiple tests connecting to the same mongo instance don't collide.
This database is not deleted on shutdown causing the disk to get full quickly.
It should be deleted and looking at the Mongodb java API the way to do it seems to be this: http://api.mongodb.org/java/2.10.1/com/mongodb/Mongo.html#dropDatabase(java.lang.String)