In short
The 3-2-1 rule means keeping three copies of your data, on two different media, with one off-site. Its current version, 3-2-1-1-0, adds one immutable or offline copy and zero errors in restore tests. In a small business it is applied with a local NAS and a copy in the cloud or at another site.
What each number means
| Number | Meaning | Example |
|---|---|---|
| 3 | Three copies of the data. | Original on the computer, a copy on the NAS and another in the cloud. |
| 2 | On two different media. | Computer drive and NAS. |
| 1 | One copy off-site. | Cloud or a NAS at another site. |
| 1 | One immutable or offline copy. | Immutable snapshots or a drive that is disconnected. |
| 0 | Zero errors on restore. | Periodic recovery tests. |
How to apply it in a small business
- Take inventory: computers, servers, shared folders, email, cloud apps and websites.
- Decide how much you can lose and how long you can be down: that sets frequency and backup type.
- Automatic local backup to a NAS for fast recovery.
- Second encrypted copy off-site: in the cloud or on a NAS elsewhere.
- Immutable snapshots to withstand ransomware.
- Failure alerts and a restore test at least every few months.
Common mistakes
- Confusing sync with backup: if a file is deleted or encrypted, sync replicates the problem.
- Thinking RAID is a backup.
- Keeping the only copy on a drive permanently attached to the same computer.
- Never checking whether the backup can be restored.