> >> syslog shows that the system has attempted to go into sleep or suspend
>
> > Can the machine be made to respond to wake-on-lan ?
>

Probably not, syslog is full of messages about disk have gone to sleep, etc.
It looks like half of the machine is asleep, half not. These desktop machines
probably cannot sleep/suspend correctly - all linux suspend code assumes laptop
hardware, probably has no drivers for desktop boards, plus desktop bios probably
does not know how to sleep without hitting all the bugs.

> > If so, setting that up *might* reduce the pain when this happens again ...

No go, a server still goes down when it happens. Revive it by special ping or by hitting
power switch makes no difference.

> Given that the issue starts with NetworkManager reporting "sleep requested": is there such a thing as "sleep-on-lan"?

Ah, yes. The famous "ping of death".

> We're not encountering this problem. Maybe because we're still not using NetworkManager...

I doubt network manager goes to sleep on it's own initiative - systemd is in agreement
and also goes to the sleep and suspend targets. I think something is spuriously activating
the sleep/suspend code inside systemd. Could be as simple as a spurious power switch hardware
glitch (on 3-4 machines? that have run sl6 for years without fault? hmm...).

-- 
Konstantin Olchanski
Data Acquisition Systems: The Bytes Must Flow!
Email: olchansk-at-triumf-dot-ca
Snail mail: 4004 Wesbrook Mall, TRIUMF, Vancouver, B.C., V6T 2A3, Canada