A presales field note about the one number that decides an object storage design — the gap between raw disk and sellable capacity — and the checks I now run before I put a terabyte figure in any proposal.

The request is always short.

“We need 100 TB of S3.”

It comes from an application team. Backups, documents, file sync, media — the workload changes, the sentence does not.

The first draft of the design usually answers it just as quickly. Four servers. Disks that add up to a round 100 TB. A line in the proposal that says “100 TB object storage”.

I ask one question before anything goes out.

“After parity, and after we stop filling at a safe level — how much of that 100 TB can the customer actually write?”

Silence. Then someone opens a calculator.

The honest answer is about 60 TB.

That’s the trap. Raw capacity does not show up as a problem on day one. The cluster comes up, the dashboard shows a big number, the first uploads land.…