Evaluating the Need for Disaster Tolerance

Disaster tolerance is the ability to restore applications and data within a reasonable period of time after a disaster. Most think of fire, flood, and earthquake as disasters, but a disaster can be any event that unexpectedly interrupts service or corrupts data in an entire data center: the backhoe that digs too deep and severs a network connection, or an act of sabotage.

Disaster tolerant architectures protect against unplanned down time due to disasters by geographically distributing the nodes in a cluster so that a disaster at one site does not disable the entire cluster. To evaluate your need for a disaster tolerant solution, you need to weigh:

Risk of disaster. Areas prone to tornadoes, floods, or earthquakes may require a disaster recovery solution. Some industries need to consider risks other than natural disasters or accidents, such as terrorist activity or sabotage.
The type of disaster to which your business is prone, whether it is due to geographical location or the nature of the business, will determine the type of disaster recovery you choose. For example, if you live in a region prone to big earthquakes, you are not likely to put your alternate or backup nodes in the same city as your primary nodes, because that sort of disaster affects a large area.
The frequency of the disaster also plays an important role in determining whether to invest in a rapid disaster recovery solution. For example, you would be more likely to protect from hurricanes that happen every season, rather than protecting from a dormant volcano.
Vulnerability of the business. How long can your business afford to be down? Some parts of a business may be able to endure a 1 or 2 day recovery time, while others need to recover in a matter of minutes. Some parts of a business only need local protection from single outages, such a node failure. Other parts of a business may need both local protection and protection in case of site failure.
It is important to consider the role the data servers play in your business. For example, you may target the assembly line production servers as most in need of quick recovery. But if the most likely disaster in your area is an earthquake, it would render the assembly line inoperable as well as the computers. In this case disaster recovery would be moot, and local failover is probably the more appropriate level of protection.
On the other hand, you may have an order processing center that is prone to floods in the winter. The business loses thousands of dollars a minute while the order processing servers are down. A disaster tolerant architecture is appropriate protection in this situation.

Deciding to implement a disaster recovery solution really depends on the balance between risk of disaster, and the vulnerability of your business if a disaster occurs. The following pages give a high-level view of a variety of disaster tolerant solutions and sketch the general guidelines that you should follow in developing a disaster tolerant computing environment.

Evaluating the Need for Disaster Tolerance

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