Backup and disaster recovery

You already copied files back in Part II. This is a different conversation: not how to copy a directory, but how to survive losing a whole server and know in advance what that costs in data and in time.

Two numbers set the frame. RPO (recovery point objective) is how much data you are willing to lose, measured in time. Copy once a day and your RPO is a day: a failure loses everything written since the last copy. RTO (recovery time objective) is how long the system may stay down while you bring it back. RPO drives how often you copy, RTO drives how fast recovery has to be. These two numbers decide everything else in the chapter.

The storage discipline comes down to the 3-2-1 rule: three copies of the data, on two different me