Comment 1 for bug 1253214

Default journalling settings for Ceilometer will allocate around 3 G. Enabling the small-files option can be used to bring this down, but may not be recommended for production environments?

See also:

https://ask.openstack.org/en/question/7358/how-to-safely-clear-ceilometer-database/
http://docs.mongodb.org/manual/tutorial/manage-journaling/
http://docs.mongodb.org/manual/reference/configuration-options/#smallfiles