3-2-1 Backup Rule
Also known as: 3-2-1 Rule
The 3-2-1 backup rule requires three copies of data on two different storage media, with one copy kept off-site.
The 3-2-1 rule is a simple guideline for backup strategy: keep three copies of your data, on two different types of storage, with one of those copies stored away from the main site. It exists because a single backup sitting next to the original disappears in the same fire, theft, flood, or ransomware attack that destroys the original.
In practice this usually means production data on the server, a second copy on a different device on the same network such as a NAS, and a third copy in the cloud or on a drive that physically sits somewhere else. The two local media should differ technically, so a fault affecting one batch of disks does not take out both copies at once.
The most common mistake is skipping the off-site copy, or updating it so rarely that it is outdated by the time it is needed. A second NAS in the same server room does not count as off-site - a fire or burglary takes out both devices together.
What it means in practice
For a business this means: if you cannot say exactly where the third, off-site copy of your data physically lives, it probably does not exist. NDVDL sets up backup strategies along this pattern and checks whether the off-site copy is actually refreshed on a regular basis.
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