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Chapter 8. Planning for Disaster

Disaster planning is a subject that is easy for a system administrator to forget — it is not pleasant, and it always seems that there is something else more pressing to do. However, letting disaster planning slide is one of the worst things a system administrator can do.

Although it is often the dramatic disasters (such as a fire, flood, or storm) that first come to mind, the more mundane problems (such as construction workers cutting cables or even an overflowing sink) can be just as disruptive. Therefore, the definition of a disaster that a system administrator should keep in mind is any unplanned event that disrupts the normal operation of the organization.

While it would be impossible to list all the different types of disasters that could strike, this section examines the leading factors that are part of each type of disaster so that any possible exposure can be examined not in terms of its likelihood, but in terms of the factors that could lead to disaster.

8.1. Types of Disasters

In general, there are four different factors that can trigger a disaster. These factors are:

  • Hardware failures

  • Software failures

  • Environmental failures

  • Human errors

8.1.1. Hardware Failures

Hardware failures are easy to understand — the hardware fails, and work grinds to a halt. What is more difficult to understand is the nature of the failures and how your exposure to them can be minimized. Here are some approaches that you can use: Keeping Spare Hardware

At its simplest, exposure due to hardware failures can be reduced by having spare hardware available. Of course, this approach assumes two things:

  • Someone on-site has the necessary skills to diagnose the problem, identify the failing hardware, and replace it.

  • A replacement for the failing hardware is available.

These issues are covered in more detail in the following sections. Having the Skills

Depending on your past experience and the hardware involved, having the necessary skills might be a non-issue. However, if you have not worked with hardware before, you might consider looking into local community colleges for introductory courses on PC repair. While such a course is not in and of itself sufficient to prepare you for tackling problems with an enterprise-level server, it is a good way to learn the basics (proper handling of tools and components, basic diagnostic procedures, and so on).


Before taking the approach of first fixing it yourself, make sure that the hardware in question:

  • Is not still under warranty

  • Is not under a service/maintenance contract of any kind

If you attempt repairs on hardware that is covered by a warranty and/or service contract, you are likely violating the terms of these agreements and jeopardizing your continued coverage.

However, even with minimal skills, it might be possible to effectively diagnose and replace failing hardware — if you choose your stock of replacement hardware properly. What to Stock?

This question illustrates the multi-faceted nature of anything related to disaster recovery. When considering what hardware to stock, here are some of the issues you should keep in mind:

  • Maximum allowable downtime

  • The skill required to make the repair

  • Budget available for spares

  • Storage space required for spares

  • Other hardware that could utilize the same spares

Each of these issues has a bearing on the types of spares that should be stocked. For example, stocking complete systems would tend to minimize downtime and require minimal skills to install but would be much more expensive than having a spare CPU and RAM module on a shelf. However, this expense might be worthwhile if your organization has several dozen identical servers that could benefit from a single spare system.

No matter what the final decision, the following question is inevitable and is discussed next. How Much to Stock?

The question of spare stock levels is also multi-faceted. Here the main issues are:

  • Maximum allowable downtime

  • Projected rate of failure

  • Estimated time to replenish stock

  • Budget available for spares

  • Storage space required for spares

  • Other hardware that could utilize the same spares

At one extreme, for a system that can afford to be down a maximum of two days, and a spare that might be used once a year and could be replenished in a day, it would make sense to carry only one spare (and maybe even none, if you were confident of your ability to secure a spare within 24 hours).

At the other end of the spectrum, a system that can afford to be down no more than a few minutes, and a spare that might be used once a month (and could take several weeks to replenish) might mean that a half dozen spares (or more) should be on the shelf. Spares That Are Not Spares

When is a spare not a spare? When it is hardware that is in day-to-day use but is also available to serve as a spare for a higher-priority system should the need arise. This approach has some benefits:

  • Less money dedicated to "non-productive" spares

  • The hardware is known to be operative

There are, however, downsides to this approach:

  • Normal production of the lower-priority task is interrupted

  • There is an exposure should the lower-priority hardware fail (leaving no spare for the higher-priority hardware)

Given these constraints, the use of another production system as a spare may work, but the success of this approach hinges on the system's specific workload and the impact the system's absence has on overall data center operations. Service Contracts

Service contracts make the issue of hardware failures someone else's problem. All that is necessary for you to do is to confirm that a failure has, in fact, occurred and that it does not appear to have a software-related cause. You then make a telephone call, and someone shows up to make things right again.

It seems so simple. But as with most things in life, there is more to it than meets the eye. Here are some things that you must consider when looking at a service contract:

  • Hours of coverage

  • Response time

  • Parts availability

  • Available budget

  • Hardware to be covered

We explore each of these details more closely in the following sections. Hours of Coverage

Different service contracts are available to meet different needs; one of the big variables between different contracts relates to the hours of coverage. Unless you are willing to pay a premium for the privilege, you cannot just call any time and expect to see a technician at your door a short time later.

Instead, depending on your contract, you might find that you cannot even phone the service company until a specific day/time, or if you can, they will not dispatch a technician until the day/time specified for your contract.

Most hours of coverage are defined in terms of the hours and the days during which a technician may be dispatched. Some of the more common hours of coverage are:

  • Monday through Friday, 09:00 to 17:00

  • Monday through Friday, 12/18/24 hours each day (with the start and stop times mutually agreed upon)

  • Monday through Saturday (or Monday through Sunday), same times as above

As you might expect, the cost of a contract increases with the hours of coverage. In general, extending the coverage Monday through Friday tends to cost less than adding on Saturday and Sunday coverage.

But even here there is a possibility of reducing costs if you are willing to do some of the work. Depot Service

If your situation does not require anything more than the availability of a technician during standard business hours and you have sufficient experience to be able to determine what is broken, you might consider looking at depot service. Known by many names (including walk-in service and drop-off service), manufacturers may have service depots where technicians work on hardware brought in by customers.

Depot service has the benefit of being as fast as you are. You do not have to wait for a technician to become available and show up at your facility. Depot technicians do not go out on customer calls, meaning that there will be someone to work on your hardware as soon as you can get it to the depot.

Because depot service is done at a central location, there is a good chance that any required parts will be available. This can eliminate the need for an overnight shipment or waiting for a part to be driven several hundred miles from another office that just happened to have that part in stock.

There are some trade-offs, however. The most obvious is that you cannot choose the hours of service — you get service when the depot is open. Another aspect to this is that the technicians do not work past their quitting time, so if your system failed at 16:30 on a Friday and you got the system to the depot by 17:00, it will not be worked on until the technicians arrive at work the following Monday morning.

Another trade-off is that depot service depends on having a depot nearby. If your organization is located in a metropolitan area, this is likely not going to be a problem. However, organizations in more rural locations may find that a depot is a long drive away.


If considering depot service, take a moment and consider the mechanics of actually getting the hardware to the depot. Will you be using a company vehicle or your own? If your own, does your vehicle have the necessary space and load capacity? What about insurance? Will more than one person be necessary to load and unload the hardware?

Although these are rather mundane concerns, they should be addressed before making the decision to use depot service. Response Time

In addition to the hours of coverage, many service agreements specify a level of response time. In other words, when you call requesting service, how long will it be before a technician arrives? As you might imagine, a faster response time equates to a more expensive service agreement.

There are limits to the response times that are available. For instance, the travel time from the manufacturer's office to your facility has a large bearing on the response times that are possible[1]. Response times in the four hour range are usually considered among the quicker offerings. Slower response times can range from eight hours (which effectively becomes "next day" service for a standard business hours agreement), to 24 hours. As with every other aspect of a service agreement, even these times are negotiable — for the right price.


Although it is not a common occurrence, you should be aware that service agreements with response time clauses can sometimes stretch a manufacturer's service organization beyond its ability to respond. It is not unheard of for a very busy service organization to send somebody — anybody — on a short response-time service call just to meet their response time commitment. This person apparently diagnoses the problem, calling "the office" to have someone bring "the right part."

In fact, they are just waiting until someone who is actually capable of handling the call arrives.

While it might be understandable to see this happen under extraordinary circumstances (such as power problems that have damaged systems throughout their service area), if this is a consistent method of operation you should contact the service manager and demand an explanation.

If your response time needs are stringent (and your budget correspondingly large), there is one approach that can cut your response times even further — to zero. Zero Response Time — Having an On-Site Technician

Given the appropriate situation (you are one of the biggest customers in the area), sufficient need (downtime of any magnitude is unacceptable), and financial resources (if you have to ask for the price, you probably cannot afford it), you might be a candidate for a full-time, on-site technician. The benefits of having a technician always standing by are obvious:

  • Instant response to any problem

  • A more proactive approach to system maintenance

As you might expect, this option can be very expensive, particularly if you require an on-site technician 24x7. But if this approach is appropriate for your organization, you should keep a number of points in mind in order to gain the most benefit.

First, on-site technicians need many of the resources of a regular employee, such as a workspace, telephone, appropriate access cards and/or keys, and so on.

On-site technicians are not very helpful if they do not have the proper parts. Therefore, make sure that secure storage is set aside for the technician's spare parts. In addition, make sure that the technician keeps a stock of parts appropriate for your configuration and that those parts are not routinely "cannibalized" by other technicians for their customers. Parts Availability

Obviously, the availability of parts plays a large role in limiting your organization's exposure to hardware failures. In the context of a service agreement, the availability of parts takes on another dimension, as the availability of parts applies not only to your organization, but to any other customer in the manufacturer's territory that might need those parts as well. Another organization that has purchased more of the manufacturer's hardware than you might get preferential treatment when it comes to getting parts (and technicians, for that matter).

Unfortunately, there is little that can be done in such circumstances, short of working out the problem with the service manager. Available Budget

As outlined above, service contracts vary in price according to the nature of the services being provided. Keep in mind that the costs associated with a service contract are a recurring expense; each time the contract is due to expire you must negotiate a new contract and pay again. Hardware to be Covered

Here is an area where you might be able to help keep costs to a minimum. Consider for a moment that you have negotiated a service agreement that has an on-site technician 24x7, on-site spares — you name it. Every single piece of hardware you have purchased from this vendor is covered, including the PC that the company receptionist uses for non-critical tasks.

Does that PC really need to have someone on-site 24x7? Even if the PC is vital to the receptionist's job, the receptionist only works from 09:00 to 17:00; it is highly unlikely that:

  • The PC will be in use from 17:00 to 09:00 the next morning (not to mention weekends)

  • A failure of this PC will be noticed, except between 09:00 and 17:00

Therefore, paying on the chance that this PC might need to be serviced in the middle of a Saturday night is a waste of money.

The thing to do is to split up the service agreement such that non-critical hardware is grouped separately from more critical hardware. In this way, costs can be kept as low as possible.


If you have twenty identically-configured servers that are critical to your organization, you might be tempted to have a high-level service agreement written for only one or two, with the rest covered by a much less expensive agreement. Then, the reasoning goes, no matter which one of the servers fails on a weekend, you will say that it is the one eligible for high-level service.

Do not do this. Not only is it dishonest, most manufacturers keep track of such things by using serial numbers. Even if you figure out a way around such checks, far more is spent after being discovered than by being honest and paying for the service you really need.

8.1.2. Software Failures

Software failures can result in extended downtimes. For example, owners of a certain brand of computer systems noted for their high-availability features recently experienced this firsthand. A bug in the time handling code of the computer's operating system resulted in each customer's systems crashing at a certain time of a certain day. While this particular situation is a more spectacular example of a software failure in action, other software-related failures may be less dramatic, but still as devastating.

Software failures can strike in one of two areas:

  • Operating system

  • Applications

Each type of failure has its own specific impact and is explored in more detail in the following sections. Operating System Failures

In this type of failure, the operating system is responsible for the disruption in service. Operating system failures come from two areas:

  • Crashes

  • Hangs

The main thing to keep in mind about operating system failures is that they take out everything that the computer was running at the time of the failure. As such, operating system failures can be devastating to production. Crashes

Crashes occur when the operating system experiences an error condition from which it cannot recover. The reasons for crashes can range from an inability to handle an underlying hardware problem to a bug in the kernel-level code comprising the operating system. When an operating system crashes, the system must be rebooted in order to continue production. Hangs

When the operating system stops handling system events, the system grinds to a halt. This is known as a hang. Hangs can be caused by deadlocks (two resource consumers contending for resources the other has) and livelocks (two or more processes responding to each other's activities, but doing no useful work), but the end result is the same — a complete lack of productivity. Application Failures

Unlike operating system failures, application failures can be more limited in the scope of their damage. Depending on the specific application, a single application failing might impact only one person. On the other hand, if it is a server application servicing a large population of client applications, the consequences of a failure would be much more widespread.

Application failures, like operating system failures, can be due to hangs and crashes; the only difference is that here it is the application that is hanging or crashing. Getting Help — Software Support

Just as hardware vendors provide support for their products, many software vendors make support packages available to their customers. Except for the obvious differences (no spare hardware is required, and most of the work can be done by support personnel over the phone), software support contracts can be quite similar to hardware support contracts.

The level of support provided by a software vendor can vary. Here are some of the more common support strategies employed today:

  • Documentation

  • Self support

  • Web or email support

  • Telephone support

  • On-site support

Each type of support is described in more detail in the following sections. Documentation

Although often overlooked, software documentation can serve as a first-level support tool. Whether online or printed, documentation often contains the information necessary to resolve many issues. Self Support

Self support relies on the customer using online resources to resolve their own software-related issues. Quite often these resources take the form of Web-based FAQs (Frequently Asked Questions) or knowledge bases.

FAQs often have little or no selection capabilities, leaving the customer to scroll through question after question in the hopes of finding one that addresses the issue at hand. Knowledge bases tend to be somewhat more sophisticated, allowing the entry of search terms. Knowledge bases can also be quite extensive in scope, making it a good tool for resolving problems. Web or Email Support

Many times what looks like a self support website also includes Web-based forms or email addresses that make it possible to send questions to support staff. While this might at first glance appear to be an improvement over a good self support website, it really depends on the people answering the email.

If the support staff is overworked, it is difficult to get the necessary information from them, as their main concern is to quickly respond to each email and move on to the next one. The reason for this is because nearly all support personnel are evaluated by the number of issues that they resolve. Escalation of issues is also difficult because there is little that can be done within an email to encourage more timely and helpful responses — particularly when the person reading your email is in a hurry to move on to the next one.

The way to get the best service is to make sure that your email addresses all the questions that a support technician might ask, such as:

  • Clearly describe the nature of the problem

  • Include all pertinent version numbers

  • Describe what you have already done in an attempt to address the problem (applied the latest patches, rebooted with a minimal configuration, etc.)

By giving the support technician more information, you stand a better chance of getting the support you need. Telephone Support

As the name implies, telephone support entails speaking to a support technician via telephone. This style of support is most similar to hardware support; that there can be various levels of support available (with different hours of coverage, response time, etc.). On-Site Support

Also known as on-site consulting, on-site software support is normally reserved for resolving specific issues or making critical changes, such as initial software installation and configuration, major upgrades, and so on. As expected, this is the most expensive type of software support available.

Still, there are instances where on-site support makes sense. As an example, consider a small organization with a single system administrator. The organization is going to be deploying its first database server, but the deployment (and the organization) is not large enough to justify hiring a dedicated database administrator. In this situation, it can often be cheaper to bring in a specialist from the database vendor to handle the initial deployment (and occasionally later on, as the need arises) than it would be to train the system administrator in a skill that will be seldom used.

8.1.3. Environmental Failures

Even though the hardware may be running perfectly, and even though the software may be configured properly and is working as it should, problems can still occur. The most common problems that occur outside of the system itself have to do with the physical environment in which the system resides.

Environmental issues can be broken into four major categories:

  • Building integrity

  • Electricity

  • Air conditioning

  • Weather and the outside world Building Integrity

For such a seemingly simple structure, a building performs a great many functions. It provides shelter from the elements. It provides the proper micro-climate for the building's contents. It has mechanisms to provide power and to protect against fire, theft, and vandalism. Performing all these functions, it is not surprising that there is a great deal that can go wrong with a building. Here are some possibilities to consider:

  • Roofs can leak, allowing water into data centers.

  • Various building systems (such as water, sewer, or air handling) can fail, rendering the building uninhabitable.

  • Floors may have insufficient load-bearing capacity to hold the equipment you want to put in the data center.

It is important to have a creative mind when it comes to thinking about the different ways buildings can fail. The list above is only meant to start you thinking along the proper lines. Electricity

Because electricity is the lifeblood of any computer system, power-related issues are paramount in the mind of system administrators everywhere. There are several different aspects to power; they are covered in more detail in the following sections. The Security of Your Power

First, it is necessary to determine how secure your normal power supply may be. Just like nearly every other data center, you probably obtain your power from a local power company via power transmission lines. Because of this, there are limits to what you can do to make sure that your primary power supply is as secure as possible.


Organizations located near the boundaries of a power company might be able to negotiate connections to two different power grids:

  • The one servicing your area

  • The one from the neighboring power company

The costs involved in running power lines from the neighboring grid are sizable, making this an option only for larger organizations. However, such organizations find that the redundancy gained outweigh the costs in many cases.

The main things to check are the methods by which the power is brought onto your organization's property and into the building. Are the transmission lines above ground or below? Above-ground lines are susceptible to:

  • Damage from extreme weather conditions (ice, wind, lightning)

  • Traffic accidents that damage the poles and/or transformers

  • Animals straying into the wrong place and shorting out the lines

However, below-ground lines have their own unique shortcomings:

  • Damage from construction workers digging in the wrong place

  • Flooding

  • Lightning (though much less so than above-ground lines)

Continue to trace the power lines into your building. Do they first go to an outside transformer? Is that transformer protected from vehicles backing into it or trees falling on it? Are all exposed shutoff switches protected against unauthorized use?

Once inside your building, could the power lines (or the panels to which they attach) be subject to other problems? For instance, could a plumbing problem flood the electrical room?

Continue tracing the power into the data center; is there anything else that could unexpectedly interrupt your power supply? For example, is the data center sharing one or more circuits with non-data center loads? If so, the external load might one day trip the circuit's overload protection, taking down the data center as well. Power Quality

It is not enough to ensure that the data center's power source is as secure as possible. You must also be concerned with the quality of the power being distributed throughout the data center. There are several factors that must be considered:


The voltage of the incoming power must be stable, with no voltage reductions (often called sags, droops, or brownouts) or voltage increases (often known as spikes and surges).


The waveform must be a clean sine wave, with minimal THD (Total Harmonic Distortion).


The frequency must be stable (most countries use a power frequency of either 50Hz or 60Hz).


The power must not include any RFI (Radio Frequency Interference) or EMI (Electro-Magnetic Interference) noise.


The power must be supplied at a current rating sufficient to run the data center.

Power supplied directly from the power company does not normally meet the standards necessary for a data center. Therefore, some level of power conditioning is usually required. There are several different approaches possible:

Surge Protectors

Surge protectors do just what their name implies — they filter surges from the power supply. Most do nothing else, leaving equipment vulnerable to damage from other power-related problems.

Power Conditioners

Power conditioners attempt a more comprehensive approach; depending on the sophistication of the unit, power conditioners often can take care of most of the types of problems outlined above.

Motor-Generator Sets

A motor-generator set is essentially a large electric motor powered by your normal power supply. The motor is attached to a large flywheel, which is, in turn, attached to a generator. The motor turns the flywheel and generator, which generates electricity in sufficient quantities to run the data center. In this way, the data center power is electrically isolated from outside power, meaning that most power-related problems are eliminated. The flywheel also provides the ability to maintain power through short outages, as it takes several seconds for the flywheel to slow to the point at which it can no longer generate power.

Uninterruptible Power Supplies

Some types of Uninterruptible Power Supplies (more commonly known as a UPS) include most (if not all) of the protection features of a power conditioner[2].

With the last two technologies listed above, we have started in on the topic most people think of when they think about power — backup power. In the next section, different approaches to providing backup power are explored. Backup Power

One power-related term that nearly everyone has heard is the term blackout. A blackout is a complete loss of electrical power and may last from a fraction of a second to weeks.

Because the length of blackouts can vary so greatly, it is necessary to approach the task of providing backup power using different technologies for power outages of different lengths.


The most frequent blackouts last, on average, no more than a few seconds; longer outages are much less frequent. Therefore, concentrate first on protecting against blackouts of only a few minutes in duration, then work out methods of reducing your exposure to longer outages. Providing Power For the Next Few Seconds

Since the majority of outages last only a few seconds, your backup power solution must have two primary characteristics:

  • Very short time to switch to backup power (known as transfer time)

  • A runtime (the time that backup power will last) measured in seconds to minutes

The backup power solutions that match these characteristics are motor-generator sets and UPSs. The flywheel in the motor-generator set allows the generator to continue producing electricity for enough time to ride out outages of a second or so. Motor-generator sets tend to be quite large and expensive, making them a practical solution only for mid-sized and larger data centers.

However, another technology — called a UPS — can fill in for those situations where a motor-generator set is too expensive. It can also handle longer outages. Providing Power For the Next Few Minutes

UPSs can be purchased in a variety of sizes — small enough to run a single low-end PC for five minutes or large enough to power an entire data center for an hour or more.

UPSs are made up of the following parts:

  • A transfer switch for switching from the primary power supply to the backup power supply

  • A battery, for providing backup power

  • An inverter, which converts the DC current from the battery into the AC current required by the data center hardware

Apart from the size and battery capacity of the unit, UPSs come in two basic types:

  • The offline UPS uses its inverter to generate power only when the primary power supply fails.

  • The online UPS uses its inverter to generate power all the time, powering the inverter via its battery only when the primary power supply fails.

Each type has their advantages and disadvantages. The offline UPS is usually less expensive, because the inverter does not have to be constructed for full-time operation. However, a problem in the inverter of an offline UPS will go unnoticed (until the next power outage, that is).

Online UPSs tend to be better at providing clean power to your data center; after all, an online UPS is essentially generating power for you full time.

But no matter what type of UPS you choose, you must properly size the UPS to your anticipated load (thereby ensuring that the UPS has sufficient capacity to produce electricity at the required voltage and current), and you must determine how long you would like to be able to run your data center on battery power.

To determine this information, you must first identify those loads that are to be serviced by the UPS. Go to each piece of equipment and determine how much power it draws (this is normally listed on a label near the unit's power cord). Write down the voltage, watts, and/or amps. Once you have these figures for all of the hardware, you must convert them to VA (Volt-Amps). If you have a wattage number, you can use the listed wattage as the VA; if you have amps, multiply it by volts to get VA. By adding the VA figures you can arrive at the approximate VA rating required for the UPS.


Strictly speaking, this approach to calculating VA is not entirely correct; however, to get the true VA you would need to know the power factor for each unit, and this information is rarely, if ever, provided. In any case, the VA numbers obtained from this approach reflects worst-case values, leaving a large margin of error for safety.

Determining runtime is more of a business question than a technical question — what sorts of outages are you willing to protect against, and how much money are you prepared to spend to do so? Most sites select runtimes that are less than an hour or two at most, as battery-backed power becomes very expensive beyond this point. Providing Power For the Next Few Hours (and Beyond)

Once we get into power outages that are measured in days, the choices get even more expensive. The technologies capable of handling long-term power outages are limited to generators powered by some type of engine — diesel and gas turbine, primarily.


Keep in mind that engine-powered generators require regular refueling while they are running. You should know your generator's fuel "burn" rate at maximum load and arrange fuel deliveries accordingly.

At this point, your options are wide open, assuming your organization has sufficient funds. This is also an area where experts should help you determine the best solution for your organization. Very few system administrators have the specialized knowledge necessary to plan the acquisition and deployment of these kinds of power generation systems.


Portable generators of all sizes can be rented, making it possible to have the benefits of generator power without the initial outlay of money necessary to purchase one. However, keep in mind that in disasters affecting your general vicinity, rented generators will be in very short supply and very expensive. Planning for Extended Outages

While a black out of five minutes is little more than an inconvenience to the personnel in a darkened office, what about an outage that lasts an hour? Five hours? A day? A week?

The fact is, even if the data center is operating normally, an extended outage will eventually affect your organization at some point. Consider the following points:

  • What if there is no power to maintain environmental control in the data center?

  • What if there is no power to maintain environmental control in the entire building?

  • What if there is no power to operate personal workstations, the telephone system, the lights?

The point here is that your organization must determine at what point an extended outage will just have to be tolerated. Or if that is not an option, your organization must reconsider its ability to function completely independently of on-site power for extended periods, meaning that very large generators will be needed to power the entire building.

Of course, even this level of planning cannot take place in a vacuum. It is very likely that whatever caused the extended outage is also affecting the world outside your organization, and that the outside world will start having an affect on your organization's ability to continue operations, even given unlimited power generation capacity. Heating, Ventilation, and Air Conditioning

The Heating, Ventilation, and Air Conditioning (HVAC) systems used in today's office buildings are incredibly sophisticated. Often computer controlled, the HVAC system is vital to providing a comfortable work environment.

Data centers usually have additional air handling equipment, primarily to remove the heat generated by the many computers and associated equipment. Failures in an HVAC system can be devastating to the continued operation of a data center. And given their complexity and electro-mechanical nature, the possibilities for failure are many and varied. Here are a few examples:

  • The air handling units (essentially large fans driven by large electric motors) can fail due to electrical overload, bearing failure, belt/pulley failure, etc.

  • The cooling units (often called chillers) can lose their refrigerant due to leaks, or they can have their compressors and/or motors seize.

HVAC repair and maintenance is a very specialized field — a field that the average system administrator should leave to the experts. If anything, a system administrator should make sure that the HVAC equipment serving the data center is checked for normal operation on a daily basis (if not more frequently) and is maintained according to the manufacturer's guidelines. Weather and the Outside World

There are some types of weather that can cause problems for a system administrator:

  • Heavy snow and ice can prevent personnel from getting to the data center, and can even clog air conditioning condensers, resulting in elevated data center temperatures just when no one is able to get to the data center to take corrective action.

  • High winds can disrupt power and communications, with extremely high winds actually doing damage to the building itself.

There are other types of weather than can still cause problems, even if they are not as well known. For example, exceedingly high temperatures can result in overburdened cooling systems, and brownouts or blackouts as the local power grid becomes overloaded.

Although there is little that can be done about the weather, knowing the way that it can affect your data center operations can help you to keep things running even when the weather turns bad.

8.1.4. Human Errors

It has been said that computers really are perfect. The reasoning behind this statement is that if you dig deeply enough, behind every computer error you will find the human error that caused it. In this section, the more common types of human errors and their impacts are explored. End-User Errors

The users of a computer can make mistakes that can have serious impact. However, due to their normally unprivileged operating environment, user errors tend to be localized in nature. Because most users interact with a computer exclusively through one or more applications, it is within applications that most end-user errors occur. Improper Use of Applications

When applications are used improperly, various problems can occur:

  • Files inadvertently overwritten

  • Wrong data used as input to an application

  • Files not clearly named and organized

  • Files accidentally deleted

The list could go on, but this is enough to illustrate the point. Due to users not having super-user privileges, the mistakes they make are usually limited to their own files. As such, the best approach is two-pronged:

  • Educate users in the proper use of their applications and in proper file management techniques

  • Make sure backups of users' files are made regularly and that the restoration process is as streamlined and quick as possible

Beyond this, there is little that can be done to keep user errors to a minimum. Operations Personnel Errors

Operators have a more in-depth relationship with an organization's computers than end-users. Where end-user errors tend to be application-oriented, operators tend to perform a wider range of tasks. Although the nature of the tasks have been dictated by others, some of these tasks can include the use of system-level utilities, where the potential for widespread damage due to errors is greater. Therefore, the types of errors that an operator might make center on the operator's ability to follow the procedures that have been developed for the operator's use. Failure to Follow Procedures

Operators should have sets of procedures documented and available for nearly every action they perform[3]. It might be that an operator does not follow the procedures as they are laid out. There can be several reasons for this:

  • The environment was changed at some time in the past, and the procedures were never updated. Now the environment changes again, rendering the operator's memorized procedure invalid. At this point, even if the procedures were updated (which is unlikely, given the fact that they were not updated before) the operator will not be aware of it.

  • The environment was changed, and no procedures exist. This is just a more out-of-control version of the previous situation.

  • The procedures exist and are correct, but the operator will not (or cannot) follow them.

Depending on the management structure of your organization, you might not be able to do much more than communicate your concerns to the appropriate manager. In any case, making yourself available to do what you can to help resolve the problem is the best approach. Mistakes Made During Procedures

Even if the operator follows the procedures, and even if the procedures are correct, it is still possible for mistakes to be made. If this happens, the possibility exists that the operator is careless (in which case the operator's management should become involved).

Another explanation is that it was just a mistake. In these cases, the best operators realize that something is wrong and seek assistance. Always encourage the operators you work with to contact the appropriate people immediately if they suspect something is wrong. Although many operators are highly-skilled and able to resolve many problems independently, the fact of the matter is that this is not their job. And a problem that is made worse by a well-meaning operator harms both that person's career and your ability to quickly resolve what might originally have been a small problem. System Administrator Errors

Unlike operators, system administrators perform a wide variety of tasks using an organization's computers. Also unlike operators, the tasks that system administrators perform are often not based on documented procedures.

Therefore, system administrators sometimes make unnecessary work for themselves when they are not careful about what they are doing. During the course of carrying out day-to-day responsibilities, system administrators have more than sufficient access to the computer systems (not to mention their super-user access privileges) to mistakenly bring systems down.

System administrators either make errors of misconfiguration or errors during maintenance. Misconfiguration Errors

System administrators must often configure various aspects of a computer system. This configuration might include:

  • Email

  • User accounts

  • Network

  • Applications

The list could go on quite a bit longer. The actual task of configuration varies greatly; some tasks require editing a text file (using any one of a hundred different configuration file syntaxes), while other tasks require running a configuration utility.

The fact that these tasks are all handled differently is merely an additional challenge to the basic fact that each configuration task itself requires different knowledge. For example, the knowledge required to configure a mail transport agent is fundamentally different from the knowledge required to configure a new network connection.

Given all this, perhaps it should be surprising that so few mistakes are actually made. In any case, configuration is, and will continue to be, a challenge for system administrators. Is there anything that can be done to make the process less error-prone? Change Control

The common thread of every configuration change is that some sort of a change is being made. The change may be large, or it may be small. But it is still a change and should be treated in a particular way.

Many organizations implement some type of change control process. The intent is to help system administrators (and all parties affected by the change) to manage the process of change and to reduce the organization's exposure to any errors that may occur.

A change control process normally breaks the change into different steps. Here is an example:

Preliminary research

Preliminary research attempts to clearly define:

  • The nature of the change to take place

  • Its impact, should the change succeed

  • A fallback position, should the change fail

  • An assessment of what types of failures are possible

Preliminary research might include testing the proposed change during a scheduled downtime, or it may go so far as to include implementing the change first on a special test environment run on dedicated test hardware.


The change is examined with an eye toward the actual mechanics of implementation. The scheduling being done includes outlining the sequencing and timing of the change (along with the sequencing and timing of any steps necessary to back the change out should a problem arise), as well as ensuring that the time allotted for the change is sufficient and does not conflict with any other system-level activity.

The product of this process is often a checklist of steps for the system administrator to use while making the change. Included with each step are instructions to perform in order to back out the change should the step fail. Estimated times are often included, making it easier for the system administrator to determine whether the work is on schedule or not.


At this point, the actual execution of the steps necessary to implement the change should be straightforward and anti-climactic. The change is either implemented, or (if trouble crops up) it is backed out.


Whether the change is implemented or not, the environment is monitored to make sure that everything is operating as it should.


If the change has been implemented, all existing documentation is updated to reflect the changed configuration.

Obviously, not all configuration changes require this level of detail. Creating a new user account should not require any preliminary research, and scheduling would likely consist of determining whether the system administrator has a spare moment to create the account. Execution would be similarly quick; monitoring might consist of ensuring that the account was usable, and documenting would probably entail sending an email to the new user's manager.

But as configuration changes become more complex, a more formal change control process becomes necessary. Mistakes Made During Maintenance

This type of error can be insidious because there is usually so little planning and tracking done during day-to-day maintenance.

System administrators see the results of this kind of error every day, especially from the many users that swear they did not change a thing — the computer just broke. The user that says this usually does not remember what they did, and when the same thing happens to you, you may not remember what you did, either.

The key thing to keep in mind is that you must be able to remember what changes you made during maintenance if you are to be able to resolve any problems quickly. A full-blown change control process is not realistic for the hundreds of small things done over the course of a day. What can be done to keep track of the 101 small things a system administrator does every day?

The answer is simple — takes notes. Whether it is done in a paper notebook, a PDA, or as comments in the affected files, take notes. By tracking what you have done, you stand a better chance of seeing a failure as being related to a change you recently made. Service Technician Errors

Sometimes the very people that are supposed to help you keep your systems running reliably can actually make things worse. This is not due to any conspiracy; it is just that anyone working on any technology for any reason risks rendering that technology inoperable. The same effect is at work when programmers fix one bug but end up creating another. Improperly-Repaired Hardware

In this case, the technician either failed to correctly diagnose the problem and made an unnecessary (and useless) repair, or the diagnosis was correct, but the repair was not carried out properly. It may be that the replacement part was itself defective, or that the proper procedure was not followed when the repair was carried out.

This is why it is important to be aware of what the technician is doing at all times. By doing this, you can keep an eye out for failures that seem to be related to the original problem in some way. This keeps the technician on track should there be a problem; otherwise there is a chance that the technician will view this fault as being new and unrelated to the one that was supposedly fixed. In this way, time is not wasted chasing the wrong problem. Fixing One Thing and Breaking Another

Sometimes, even though a problem was diagnosed and repaired successfully, another problem pops up to take its place. The CPU module was replaced, but the anti-static bag it came in was left in the cabinet, blocking the fan and causing an over-temperature shutdown. Or the failing disk drive in the RAID array was replaced, but because a connector on another drive was bumped and accidentally disconnected, the array is still down.

These things might be the result of chronic carelessness or an honest mistake. It does not matter. What you should always do is to carefully review the repairs made by the technician and ensure that the system is working properly before letting the technician leave.



And this would likely be considered a best-case response time, as technicians usually are responsible for territories that extend away from their office in all directions. If you are at one end of their territory and the only available technician is at the other end, the response time will be even longer.


UPS technology is discussed in more detail in Section Providing Power For the Next Few Minutes.


If the operators at your organization do not have a set of operating procedures, work with them, your management, and your users to get them created. Without them, a data center is out of control and likely to experience severe problems in the course of day-to-day operations.

  Published under the terms of the GNU General Public License Design by Interspire