Desc: Typical setups for big servers
File: big-servers.txt
Date: 14 March 2004
Auth: Russell Kroll <rkroll@exploits.org>

By using multiple MONITOR statements in upsmon.conf, you can configure
an environment where a large machine with redundant power monitors
multiple separate UPSes.  For the examples in this document, I will use 
a HP NetServer LH4 in the stock configuration with 3 of the 4 power 
supplies installed.

Compared to some systems, the LH4 is unusual because it has common power
cords for multiple power supplies.  One cord feeds two supplies.   As a
result, each UPS drives two of the power supplies, so losing that UPS 
affects both at the same time.

For our examples, UPS "Alpha" is connected to the cord that drives the
two power supplies, and ups "Beta" is connected to the other with just a
single power supply.

Since the server needs two supplies to stay alive, we can lose "Beta" at
any time, but losing "Alpha" is a showstopper.  We make a note of them
when configuring upsmon:

	MONITOR ups-alpha@myhost 2 monuser mypass master
	MONITOR ups-beta@myhost 1 monuser mypass master
	MINSUPPLIES 2

With that configuration, upsmon will only shut down when Alpha reaches
a critical (on battery + low battery) condition, since Beta by itself
is not enough to keep the system running.  Beta, on the other hand,
can go up and down as much as it wants and nothing will happen as long
as the other one keeps working.

The MINSUPPLIES line tells upsmon that we need at least 2 power supplies
to be receiving power from a good UPS (on line or on battery, just not
on battery and low battery).

Adding redundancy
=================

The above example assumed the stock hardware configuration.  Let's say
we have successfully cajoled the boss into providing the money for a 
fourth power supply.  Now both power cords run two power supplies, and
the system is maxed out in terms of redundancy.

The upsmon.conf must be updated to reflect this:

	MONITOR ups-alpha@myhost 2 monuser mypass master
	MONITOR ups-beta@myhost 2 monuser mypass master
	MINSUPPLIES 2

Beta is now driving two power supplies, so we upgrade its power value.
This means that either Alpha *or* Beta can totally shut down and the
server will be able to keep running.  

As an added bonus, this means you can move a running server from one UPS
to another without bringing it down since the minimum power will be
provided at all times.

Other configurations
====================

There are a lot of ways to handle this and they all come down to how many
power supplies, power cords and independent UPS connections you have.  A
system with a 1:1 cord:supply ratio has more wires stuffed behind it, but
it's much easier to move things around since any given UPS drives a smaller
percentage of the overall power.
