Thanks. I am fairly certain (?) at this point that NFS is the culprit. I had a lot of trouble unmounting one of my NFS directories. Eventually I resorted to rebooting, at which point it went into a permanent hang until a reboot was forced via hypervisor.

Is this particular situation, where NFS causes nginx to shut down, specific to nginx? We just switched from apache to nginx at the start of the year. I didn't have NFS problems before that. I don't know if that's coincidence or not.

At any rate, my takeaway from this: nginx + NFS = bad.