Following up with this, after much appreciated assistance (thanks Ben, Leif and Yongming!), the system started behaving once I rebooted to something other than Ubuntu 13.10’s 3.11.0-12 kernel. Why I didn’t try that earlier, I don’t know.
Nearly twenty million successful requests later, I can confirm there are no stuck cache reads, and no users reporting stalled sessions. I’m unable to replicate the behaviour myself, even being able to trigger it on the first query. David Jericho Senior Systems Administrator AARNet Pty Ltd
