You seem like you know something about rethink. I do not, and couldn't find this information but maybe you know?
One irritating feature of Mongo (and I suppose other db's too; orientdb does this) is that once it grabs disk space, it never lets it go. It'll reuse that space, but the OS will never see it again. (EG: If I store 10GB of stuff, and delete 9GB of it, the OS still sees all 10GB of disk used).
