How Backups Work

The difference between having a copy and having a backup.

A backup is only worth what you can actually restore from it. Most advice about backups is really advice about making sure the copy exists, is separate, and still works when you need it.

"I have a copy" is not enough

Two copies sitting on the same disk — or in the same room — tend to fail together. A spilled drink, a failed drive, a stolen laptop, or a ransomware infection can remove both at once. The point of a backup is that it survives the event that destroys the original.

Full, incremental and differential

• Full — a complete copy of everything. Simple and self-contained, but large and slow to run often.
• Incremental — only what changed since the last backup of any kind. Small and fast, but restoring means replaying a chain of increments.
• Differential — only what changed since the last full backup. Larger than an incremental, but restoring needs just the full plus the latest differential.

Most backup tools combine these — for example, a full copy every week with incrementals in between.

Snapshots

A snapshot captures the state of a system at a point in time, usually without copying everything immediately. Snapshots are excellent for quickly undoing a bad change, but they often live on the same storage as the data, so they are not a substitute for a separate backup.

The 3-2-1 rule

A simple, durable guideline: keep 3 copies of your data, on 2 different kinds of media, with 1 of them off-site. It is easy to remember and it covers the most common ways people lose everything at once.

Test your restores

An untested backup is a guess. Schedule a periodic restore of a file — or better, a whole system — into a safe location, and confirm it opens and works. This is the single most valuable habit in all of backup practice.

Things that quietly go wrong

• Backups that silently stop running and nobody notices for months.
• Ransomware encrypting the backup along with the original, because it was connected.
• A backup stored next to the original, sharing its worst day.