[Linux-cluster] 2-node tie-breaking

[Steven Whitehouse]

Hi,

On Tue, 2008-02-12 at 09:50 -0500, Lon Hohberger wrote:
> On Fri, 2008-02-08 at 15:32 +0000, gordan at bobich.net wrote:
> 
> > I just had another thought. Most motherboards these days are ATX, which 
> > means ATX type "short-pins-to-power-on" power switches. That means that as 
> > a _REALLY_ cheap solution I could just get something like a small relay 
> > switch and wire it into the serial port. When a pin on RS232 goes high 
> > (e.g. DTR), it activates the switch. I think it would be pretty reliable, 
> > and the total cost of components would be pennies. The fence agent would 
> > also be a total of about 10 lines of code, too. :)
> 
> It'd have to be a 'press-and-hold' sort of thing.  E.g. either activate
> or deactivate DTR for 5+ seconds. ;)
> 
I did something similar in the early days of Sistina. I used a (ISA
bus!) lab card with 12 relays on it, one of which was connected across
the reset switch of my test machine. Together with a serial console that
gave me all the control that I generally needed when working remotely.

Its relatively unlikely that an RS-232 port would supply enough current
to drive a relay directly, but you might find a suitable alternative in
an opto-isolator, and that would also have the advantage of not being
inductive and thus it won't potentially put a surge onto your
power-rails in case your decoupling isn't 100%. Its also cheaper.

Steve.