What is the 3-2-1 Backup Rule?
The 3-2-1 backup rule is a concept made famous by photographer Peter Krogh. He basically said there are two types of people: those who have already had a storage failure and those who will have one in the future. Its inevitable. The 3-2-1 backup rule helps to answer two important questions: how many backup files should I have and where should I store them?
The 3-2-1 backup rule goes as follows:
- Have at least three copies of your data.
- Store the copies on two different media.
- Keep one backup copy offsite.
- Create at least THREE different copies of your data
Yes, I said three copies. That means that in addition to your primary data, you should also have at least two more backups that you can rely on if needed. But why isn’t one backup sufficient you ask? Think about keeping your original data on storage device A and its backup is on storage device B. Both storage devices have the same characteristics, and they have no common failure causes. If device A has a probability of failure that’s 1/100 (and the same is true for device B), then the probability of failure of both devices at the same time is 1/10,000.
So with THREE copies of data, if you have your primary data (device A) and two backups of it (device B and device C), and if all devices have the same characteristics and no common failure causes, then the probability of failure of all three devices at the same time will be 1/1,000,000 chance of losing all of your data. That’s much better than having only one copy and a 1/100 chance of losing it all, wouldn’t you say? Creating more than two copies of data also avoids a situation where the primary copy and its backup copy are stored in the same physical location, in the event of a natural disaster.
- Store your data on at least TWO different types of media
Now in the last scenario above we assumed that there were no common failure causes for all of the devices that contain your precious data. Clearly, this requirement is much harder to fulfill if your primary data and its backup are located in the same place. Disks from the same RAID aren’t typically independent. Even more so, it is not uncommon to experience failure of one or more disks from the same storage compartment around the same time.
This is where the #2 comes in 3-2-1 rule. It is recommended that you keep copies of your data on at least TWO different storage types. For example, internal hard disk drives AND removable storage media such as tapes, external hard drives, USB drives, od SD-cards. It is even possible to keep data on two internal hard disk drives in different storage locations.
- Store at least ONE of these copies offsite
Believe it or not, physical separation between data copies is crucial. It’s bad idea to keep your external storage device in the same room as your primary storage device. Just ask the numerous companies that are located in the path of a tornado or in a flood zone. Or what would you do if your business caught fire? If you work for a smaller company with only one location, storing your backups to the cloud would be a smart alternative. Tapes that are stored at an offsite location are also popular among companies of all sizes.
Every system administrator should have a backup. This principle works for any virtual environment; regardless of the system you are running, backup is king!