[galaxy-dev] cannot "Delete Permanently"

I installed a local galaxy server, did some calculation. Now I want to clean datasets for some space. I tried "Delete Permanently" on both datasets and histories, none of them worked. The datasets and histories will be marked as "deleted", but it will not be really deleted even I am using "Delete Permanently". They will not be really removed, and space are not released. Sometimes it will give me a message "this feature is not enabled", sometimes it won't. Any one know how to enable "Delete Permanently" feature? Thanks, Fenglou