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Blackout A total failure of the power supplied to the server.
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Spike A spike is a very short (usually less than a second) but very intense increase in voltage. Spikes can do irreparable damage to any kind of equipment, especially computers.
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Surge Compared to a spike, a surge is a considerably longer (sometimes many seconds) but usually less intense increase in power. Surges can also damage your computer equipment.
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Sag A sag is a short-term voltage drop (the opposite of a spike). This type of voltage drop can cause a server to reboot.
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Brownout A brownout is a drop in voltage that usually lasts more than a few minutes.
Many of these power-related threats can occur without your knowledge; if you don't have a UPS, you cannot prepare for them. For the cost, it is worth buying a UPS, if for no other reason than to sleep better at night.
Disaster Recovery
Even the most fault-tolerant networks will fail, which is an unfortunate fact. When those costly and carefully implemented fault-tolerant strategies do fail, you are left with disaster recovery.
Disaster recovery can take on many forms. In addition to real disaster, fire, flood, theft, and the like, many other potential business disruptions can fall under the banner of disaster recovery. For example, the failure of the electrical supply to your city block might interrupt the business function. Such an event, although not a disaster per se, might invoke the disaster recovery methods.
The cornerstone of every disaster recovery strategy is the preservation and recoverability of data. When talking about preservation and recoverability, we are talking about backups. When we are talking about backups, we are likely talking about tape backups. Implementing a regular backup schedule can save you a lot of grief when fault tolerance fails or when you need to recover a file that has been accidentally deleted. When it comes time to design a backup schedule, there are three key types of backups that are usedfull, differential, and incremental.