For one company, Monday mornings often required frantic activity to restart three Novell servers. As reported by system logs, something was shutting them down abnormally around 4:00 P.M. each Friday. After many futile investigations, the company eventually found that a nearby factory tested its backup power generators late Friday afternoons. The resulting brownouts and spikes injected into the power grid were sufficient to stall these particular servers.
For another company, Windows servers promptly hung around 12:00 noon during workdays. Bewildered IT staff learned in dismay that the servers were on the same power circuit with the elevators. The lunch hour activities of the elevators were injecting power brownouts serious enough to hang the Windows servers.
Both of these cases could lead to file corruption resulting from these restarts and hangs. While this can usually be easily fixed on a file server (by restoring from backup), the problem can be much more damaging on an application or database server and may lead to extended downtime while trying to complete a proper repair.
As these two anecdotes illustrate, managing power so that your data centre systems can receive a clean and continuous supply of electricity is an absolutely critical part of system administration. You must be able to select the appropriate uninterruptible power supply (UPS) and/or electrical generator units for your organisation. You should also be able to use appropriate -power" terminology with your suppliers so they can help you meet your needs.
Power terminologies
Here are some power terms that you should know to get started:
- Brownout/Sag: Momentary drop in voltage
- Spike/Surge: Increase in voltage usually for a split moment
- Line conditioning: Inject power to minimise brownout or absorb power to reduce spikes
- Power conditioning: Ensures consistent supply of power
- Transformer: Converts AC voltage to another
- Power inverter: Converts between AC and DC
- Power: Measured in watts, W
- Current: I, measured in amps, A
- Volt-amp: VA, where 1 watt = 1.4 VA
- Line noise: Due to electro magnetic interference (EMI) and radio frequency interference (RFI)
- Uninterruptible power supply (UPS): Used to provide temporary constant power when public power supply fails
- Electrical generator: Used to provide constant power; can be powered by diesel, liquid propane gas, natural gas, or even steam
- Hertz (also known as cycle): Get the cycle wrong and see motors burn out before your eyes even if you got the voltage right
- Public power: Electrical power from public utilities
- Local power: Electrical power from power generators
UPS
There are two types of uninterruptible power supplies, Standby (or Offline) and Online. The distinction boils down to what actually powers attached devices, as illustrated in Table A.
Comparison of a Standby UPS and an Online UPS
So what's a good UPS for your organisation? You need to decide which devices are going to be attached to the UPS. If your budget allows it, get as many UPS units as possible with as much battery power as possible. If you want to eliminate a single point of failure, it's better to use many smaller UPS units than a single large unit.
Most UPS boxes are rated in VA, KVA, Watts, or Kilowatts (KW). Use the volt-amp formula in the list above and start adding up the wattage requirements of each device you want to attach to each UPS.
Sample exercise for calculating UPS power
Let's say that you want to attach a server and three hubs to a UPS. Check the specs and add up the total wattage needed. Multiply the wattage figure by 1.4 and you get the VA requirement. Get a UPS that is rated above this VA. I recommend a bigger unit if you plan to add more devices. Try to dedicate one UPS per critical piece of equipment, such as server, router, or a switch, to avoid a single point of failure.
Another question to consider is how long you want the UPS to keep your attached devices running. Your UPS will stay operational longer if the attached devices draw less power.
For critical equipment, make sure that your UPS units support line conditioning. Line noise filtering is usually expressed in decibel levels (dB) at specific frequencies (KHz or MHz). Since this feature is certainly desirable, get units that offer noise filtering at higher decibel levels.
Cost can be an issue, but you should still try to get the best units you can. If you go cheap, your organisation may suffer due to a severe lack of computing support. If you want to maintain superior uptime, get backup UPS units as well. Figures A and B show what typical UPS units look like.
APC large UPS
APC integrated UPS
6%
1%
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