3-2-1 rule
Keep three copies of data on two different media, with one copy offsite.
It is deliberately simple and has held up for decades. The production data plus two backups, stored on different technologies, with one somewhere a local disaster cannot reach.
Modern variants add a fourth element: one copy immutable or offline, because ransomware crews now target backup infrastructure specifically before triggering encryption.
Where this comes up
3-2-1 rule sits inside our backup and disaster recovery work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.
Related terms
Backup A separate copy of data kept so it can be restored after loss, corruption, deletion, or encryption. Offsite backup A backup copy kept at a different physical location from the systems it protects. Immutable backup A backup that cannot be modified or deleted for a defined period, even by an administrator. Air-gapped backup A backup copy kept physically or logically disconnected, unreachable from the production network.