High Availability and Disaster Recovery : They ARE not the same

I have run into people over the years who don’t understand that there is a difference between the two.

I’m going to use a more common technology to explain the difference – run-flat tires. For those not familiar with these they are tires which, even with a puncture of upwards of an inch, can be used for up to 50 miles without being changed out.

Let’s say we stick a set of these on a car. The car now has what could be called ‘high availability’ tires. Run over a nail? No problem – you can keep going for a while without having to worry about losing control of the car. The run-flat tires provide the car with high availability – being able to continue to use the car while in a degraded condition. In the computer world this would translate to various technologies – RAID configurations, clustered servers, replicated configurations, etc. Again, the goal is to provide a level of functioning while in a degraded condition.

How is this different from disaster recovery?

Still using our run-flat tires we’re going to expand the discussion. Say, instead of a nail you run over a  shard from a 2×4 that puts a five inch gash in the tire. The high availability of the run-flat tires is now useless. Now you have to have a spare tire. This is the difference between high availability and disaster recovery : disaster recovery allows you get going again after a total failure. In our tire example the spare tire is what in the computer world is called backup.

Translating this pair of examples into computer world jargon – the high availability is usually best known in clustering and RAID, disaster recovery in backups. HA can protect against system outages, but if Joe Disgruntled decides to delete a bunch of things HA will do no good while DR via backups will.

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