$ ps -C mongod -f | grep directoryperdb
ubuntu   15776     1  0 Dec20 ?        00:00:07 mongod --fork --logpath mongod.log --directoryperdb
$ mount | grep /data/db
/dev/xvdg on /data/db/foo type ext4 (rw,noatime)
$ mongo
MongoDB shell version: 2.2.1
connecting to: test
> use foo
switched to db foo
> db.c.insert({})
> db.dropDatabase()
{
	"errmsg" : "exception: delete data files with a directoryperdb caught exception exception: boost::filesystem::remove: Device or resource busy: \"/data/db/foo\"",
	"code" : 14043,
	"ok" : 0
}
>