Any server
Backup Storage Plan
Retention policies are written in days and paid for in gigabytes, and the conversion between them surprises people. Fourteen daily copies of a growing world is not fourteen times today's size — it is more, because the world grows underneath the policy.
How this is calculated
Full copies are the simple case and the expensive one: every backup is the whole world, and since the world grows across the retention window the average copy sits midway between today's size and the last day's. Multiply that average by the number of copies you keep. Incremental is the other shape: one complete copy plus the accumulated daily changes, which for a slowly growing world is dramatically smaller. Compression is applied last, as a straight percentage off the total.
The arithmetic
- size after N days = current + growth × N
- average full copy = (current + size after N days) ÷ 2
- full total = average copy × copies × (1 − compression)
- incremental total = (size after N days + growth × N) × (1 − compression)
What people get wrong
The number that dominates the result is growth per day, and it is the one people guess at. Measure it instead: check the world directory's size on two consecutive days. The second trap is backup frequency — going from one copy a day to four does not cost a little more, it costs four times as much, because each copy is a full one unless you have specifically set up incremental.