Desc: Typical setups for big servers
File: big-servers.txt
Date: 30 April 2003
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 out of 4 power
supplies installed.

The HP is unusual (compared to some systems, at least) in that it has
common power plugs for a given side of the server.  One plug drives two
power supplies.  This means that losing one UPS shuts down both of them.
Here's the arrangement for our example:

UPS "Alpha" is running the plug with two supplies in it.
UPS "Beta" is running the other plug, with the single 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
=================

In this case, the server is in the stock setup as received from HP, which 
has 3 out of 4 installed.  Let's say some extra money is found in the budget
and we get a fourth supply and install it.  The upsmon.conf must now be
edited 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.
