MongoDB's heartbeat function does not monitor the health of the disk writes / reads. So in case the underlying disks on the primary node are having problems MongoDB will not switch primary.

I would like a feature in the heartbeat function that includes health checking the read/write performance. It would probably be good if this more extensive heartbeat function is optional. See the discussion on mongodb-user maillist https://groups.google.com/forum/?fromgroups#!starred/mongodb-user/gY7r3f-yz0k.

Right now the only option for us when a node has disk problems is to stop the mongod process in order to force a change of primary node.