updated 04:50 pm EDT, Mon June 27, 2011
Bug appears to affect auto-restart on some Xserves
A bug discovered in the Mac OS X 10.6.8 Server update is causing the hardware monitoring daemon, hwmond, to consume up to 95 percent of CPU power on a variety of Xserves, according to an Apple forum thread. Member Arminhempel reports a CPU load up to 70 percent after every reboot on several Xserves he manages; Jean-Serge Remy has an Xserve Xeon showing an 84 percent hit, while InfraredAD is experiencing a 95 percent load on a 2008 quad-core Xserve, essentially wiping out an entire core. Both the combo and delta versions of the update have generated errors and, so far, the only reliable way to fix the issue is a downgrade to Mac OS X 10.6.7.
The update is also in some cases triggering a problem in which Xserves sit idle instead of automatically restarting. Some forum members who have tried restarts to rid themselves of hwmond issues have seen their servers first fail to shut down on command, then fail to restart. InfraredAD was forced to use the physical power button to shut his system down, after menu and command-line efforts failed. In four hours of leaving the server idle, the machine never automatically restarted.
The hwmond daemon can itself be shut down, but leaves a server without hardware monitoring, disabling the front LEDs that update the operational conditions of the machine. More importantly killing hwmond ends reporting, a very dangerous situation for administrators running RAID volumes or mission-critical servers.