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Clean Reliable Power

by Lloyd Borrett
Technical Cornucopia, August 1989

It's quite simple really, our computers run on electricity. Without it they are worthless. But how many of us have ever taken the time to think through the implications of that one simple fact.

Power isn't always clean or reliable

If you work around computers for long enough you learn to appreciate the damage that can be caused by power fluctuations. To help avoid data loss and circuitry damage, some users install such protective devices as surge protectors, line conditioners and other filtering devices. Unfortunately, doing so may provide a false sense of security. While these units offer protection against voltage spikes and surges, they fail to address the most common type of power disturbance loss of power.

Alternating current power line sags better known as brownouts typically account for over 80 percent of all power disturbances. Blackouts cause an additional 5 percent of power problems. What this means is that power protection solutions, while important to system integrity, are irrelevant for over 90 percent of electrical problems.

Realisation of this has led to a dramatic expansion in the development and use of backup power systems (BPS). Essentially there are two types of backup power systems the Un-interruptible Power Source (UPS) and the Standby Power Source (SPS). The difference is that a UPS always supplies its output power from its storage bank of batteries.

The SPS supplies its output power from the mains supply and switches to the storage bank if the incoming power begins to deviate from the accepted voltage range. Provided the SPS switches over fast enough to keep your equipment running it can be considered to be the same as a UPS. Typically, a SPS costs less than a UPS.

A good backup power system combines the benefits of surge suppressors with the added protection of a redundant power source. During normal operation, a good quality BPS filters and conditions incoming power. A BPS supplies AC power during brownouts or blackouts and keeps a system up and running during power outages.

In most cases, power interruptions are momentary and users continue working as if nothing has happened. When the power is off for an extended period and the battery power is at risk of drainage, the BPS alerts users in ample time for them to save their work and close down the system.

Network susceptibility

Networked computer systems are, not surprisingly, much more susceptible than single-user systems to data loss due to power failures. In a typical LAN, a single file server is normally used to maintain files on a hard disk. If a power failure occurs when the server is updating a disk directory or a file-allocation table, the results can give the network manager nightmares!

In addition, many network operating systems hold the hard disk directory in random-access memory, a somewhat volatile state, and only periodically update the hard disk itself. If a directory on the hard disk had not been updated at the time of the power failure, it may be impossible to recover whatever data was still in RAM.

Potential for costly losses

Lost network information can obviously be costly. Large network databases contain everything from customer accounts to financial data, as well as irreplaceable creative work. Damage to this mass storage can mean lost business, billing errors or scheduling delays.

And consider the intangible losses from damaged data: lost productivity, energy wasted on data re-entry, dips in morale, possible lost confidence in the computer system and worse yet, in the system managers! It can even adversely affect a company's image among its customers.

LAN managers are thus increasingly turning to backup power systems to protect against data loss. But while a BPS on the file server can eliminate some of the risks, problems still remain. A BPS alone, for instance, cannot shut down a system by itself or safely warn all network connections of an impending shutdown.

Ideal conditions

A safe system shutdown depends on the presence of a system supervisor at the time of the power failure and relies on the following sequence of events taking place. First, the supervisor must realise a power failure has occurred and notify the workstations to give network users adequate time to save their files and log off the system. Then the network server must be physically shut down.

But these are ideal conditions, and rarely met in the real world. What happens, for instance, when a server is in operation around the clock without the benefit of a trained supervisor? Or when remote workstations are not made aware of a power disturbance at the file server?

Intelligent BPS

To address these questions and provide for orderly system shutdown, some network and BPS vendors have made several advancements in both hardware and software. Some BPS manufacturers, recognizing the need to interface directly with the machines they support, have added external signals that each user's equipment can read. The signals, communicated through standard serial or parallel ports, indicate when a power disturbance has occurred (i.e. when the BPS is providing power to the system without mains support) and when the BPS' battery reserve is dangerously low.

Because the BPS is already capable of supplying power to the file server during power fluctuations and outages, all that is required is a foolproof system to make the processor aware of the power problem. In general, this strategy involves the CPU reading the BPS status at periodic intervals.

When this status is normal, indicating acceptable mains power, nothing is done. When the server receives a signal indicating a failed power line, however, it should warn the user that the system is operating on battery power. This is typically done via a broadcast message to remote workstations.

If mains power does not resume within an acceptable amount of time (depending on the battery's capacity and system load), users should be advised and a system shutdown should be automatically executed.

A new awareness

The importance of automatic data protection in the event of power loss has increased as businesses with less experienced personnel depend more on computers. This new awareness has led to an increase in the number of applications programs and operating systems, including 3Com's 3Plus, designed to monitor the status of a BPS.

Systems using 3Com's 3S/401 or 3S/402 servers, for instance, can monitor the BPS via a direct cable link between a compatible BPS and the asynchronous serial port (no boards or other hardware are necessary). The BPS monitoring function, automatically activated when the system is powered up, runs as a background process invisible to users during normal operation.

Then, when a server detects power failure, a new sequence of events takes place, without human intervention. The monitoring system broadcasts a status display on the file server and the workstations are notified that the system is running on BPS power. After a user-specified period of time (generally 10 to 20 minutes), the operating system initiates the shutdown routine, writing all data in memory to disk and shutting itself down.

In this way, users at remote workstations which may not be subject to power fluctuations affecting the server will have adequate time to store their data before the system shuts down.

A supervisor need not be present during the process, so that a shock to the computer system need not be a shock to the network administrator.

Last modified: Saturday, 15 October 2011


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