the 3-2-1 backup rule
expressionops cultureIT support
The backup rule of thumb: three copies of the data, on two different media, with one copy off-site.
Simple enough to remember mid-disaster, and it survives most single failures: device death, site loss, even ransomware if the off-site copy is offline or immutable. The modern extension is 3-2-1-1-0: one copy offline, zero restore errors.
Three copies of the data, on two different media types, with one held offsite. The rule is old, it predates cloud storage entirely, and it survives because each number defends against a different failure that the others do not.
Three copies means a single corruption does not exhaust your options, since the production copy and one backup can both be bad. Two media types defends against a systemic fault in one technology, a firmware defect affecting an entire drive model, or a format nothing can read any more. One offsite copy defends against everything that is geographic: fire, flood, theft, and the building being inaccessible.
Ransomware forced an addition, usually written as one immutable or offline copy, because an attacker with administrative access will delete backups they can reach, and a backup system reachable from the network is a backup system inside the blast radius. The other half of the rule that nobody writes down is that a backup nobody has restored is not a backup, it is a belief about a backup, and the only way to convert one into the other is to actually try it.
Also known as: 3-2-1 rule, 3-2-1-1-0