Explain like I'm five · Security

What is a Backup?

Backups, told as a story about copying the precious chest and keeping the copy far away.

the only onelose this chest and the castle starts from nothing

The precious chest is the only one.Books, ledgers, photos. Lose this chest and the castle starts from nothing.

firefloodthe thief's lockI threw it out by mistakethe last one is the most common

Chests get lost in many ways.Fire, flood, the thief's lock — and most often of all, me throwing it out by mistake.

the originalwooden chestiron chestdepot in another town3 · 2 · 1three copies · two kinds · one far awayburnt, soaked or locked, another one remains

A backup is a copy of the chest, kept somewhere else.Make three. Two different kinds, one far away. That's 3-2-1.

Three copiesone can always be lost
Two kindsa wooden chest and an iron one
One far awaya fire here leaves the next town fine
Kept apartout of the thief's reach
every nightcopy only what's newonce a month, open itunopened, a copy might as well not exist

Copy every day, and open it now and then.A copy you never open might as well not exist. Once a month, open it and check.

copied last nightthis afternoon?this afternoon's pictures aren't in it yetonly up to the last copycopy it with the bug, and the copy has the bug3 days ago2 days agoyesterdaytodayso several days' copies are kept apart

A copy holds only what was there when it was made.This afternoon's pictures aren't in it yet. Copy a chest with a bug inside and the copy has the bug — so several days' copies are kept apart.

In one breath

A backup = the precious chest copied three times, in two kinds, one kept far away — and opened now and then to be sure.

The 3-2-1 rule. Decide first how much you can afford to lose (RPO) and how fast you must be back (RTO). It's also the real answer to ransomware.

When grown-ups say it

3-2-1
Three · two · one. Three copies, two media, one off-site. The easy-to-remember minimum.
Full · incremental
Everything · only what changed. A whole copy now and then; every day, just the new bits.
Snapshot
A photo of that moment. Fast, but it lives in the same room — and burns with it.
Air-gapped · immutable
The chest kept apart. Out of the thief's reach. → the thief's lock on every chest
RPO (recovery point objective)
How much may be lost. You go back to the last copy. An hour? A day?
RTO (recovery time objective)
How fast you must be back. How long it takes to fetch the spare and put it back.
Restore test
Opening it. Having a spare and being able to open it are different things.
Retention · versions
Several days' copies. So you can go back to a chest from before the bug got in.
Next story
The Letter Nobody Can Read Without the Key →
Want another term as a picture book? Request →