Over the past month, I have observed that my mercury instance has stopped twice; about 14 days apart. In both cases the culprit appears to be that the hudson log is filling. From my syslog:
hudson: failed to write(client_errfd = 5): File too large
If I move the /var/log/hudson/hudson.log and restart the server, everything comes back properly.
For the time being, I'm turning Hudson log level to off. Here's a link that might help someone else: http://wiki.hudson-ci.org/display/HUDSON/Logging.
Any other tweaks I should consider?