Backup (3-2-1 rule)

Three copies of data, on two kinds of storage, one of them off-site, plus regular restore tests.

The rule protects against different failures at once: a deleted table, a broken disk, a lost account or ransomware. Today one copy is often immutable or offline, so an attacker with admin rights cannot delete it. A backup that has never been restored is a hope, not a backup: formats change, keys get lost, restores take longer than anyone assumed.

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Related terms

RPO and RTO: How much data you can afford to lose (RPO) and how long you can be down (RTO) after a disaster.
Runbook: A short document that tells someone else how to run a system: where it lives, how to deploy, how to recover.
Postmortem (blameless): A written review after an incident: what happened, why, how it was handled and what changes so it does not repeat.

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