Since upgrading to 3.6 (yes I know!) we have found that dropped collections remain in pending on secondaries.
We’re running a primary-secondary-arbiter setup. The primary immediately frees disk space for dropped collections. The secondary however keeps the dropped collections in pending, and doesn’t free disk space.
We’re working around the problem by periodically restarting mongod on the secondary as drop-pending collections are removed on shutdown.
Could this be a configuration problem? We’re mostly using default settings (WiredTiger with journal enabled).