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Backups and 3-2-1: three copies, two kinds of media, one of them somewhere else

A backup is a copy you can restore from. 3-2-1 is the rule that makes it survive: three copies of the data, on two different kinds of media, with one copy off-site. And a backup that has never been restored is a hope, not a backup.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

The office floods on a Sunday. The server and the backup drive sitting on top of it are both under a foot of water. The nightly backup ran perfectly for three years and every copy was in the same room. 3-2-1 exists for that Sunday: the third copy, in a different building or in the cloud, is the only one that matters that day.

How to think about it

Count the copies, including the live one. Count the kinds of media: the internal disk and an external disk are two media; two folders on the same disk are one. Ask where each copy physically is; if all of them share a roof, there is no off-site copy. Then test a restore of one real file to a separate location, open it, and write the date you did that in the ticket.

Worked example

Copy 1: the live files on the server's disk
The working copy counts. It is one of the three.
Copy 2: nightly backup to an external drive in the server room
A second copy, on a second kind of media. Same roof.
Copy 3: weekly copy to the cloud service
Off-site. This is the one the flood cannot reach.
Restore test: one contract restored to a test folder, opened, correct. Dated in the ticket
The rule is satisfied on paper. The restore test is what makes it true.

Your turn

Write how many of the three copies must be off-site under 3-2-1.

3 copies, 2 kinds of media,  copy off-site

The trap

Counting a copy in the same building, or on the same disk, as a second copy. A folder called 'backup' on the same disk dies with the disk. The rule counts places and media, not folder names.

Practise backups and 3-2-1 on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.