Forgot The Third Copy

A teammate sizes the storage for a clickstream warehouse with this script and takes the printed number to the capacity meeting.

EVENTS_PER_DAY = 2_400_000_000
BYTES_PER_EVENT = 380
RETENTION_DAYS = 90

total_bytes = EVENTS_PER_DAY * BYTES_PER_EVENT * RETENTION_DAYS
print(round(total_bytes / 1e12, 1), "TB")
# prints: 82.1 TB

The cluster is a three-replica quorum store. Six weeks after provisioning 90 TB the cluster is full. What did the script leave out, and what is the real figure?

Compression. Columnar storage would shrink 82.1 TB to roughly 20 TB, so the provisioning was too generous, not too small.

The replication factor. Three copies of 82.1 TB is roughly 246 TB.

The unit. Dividing by $10^{12}$ gives decimal TB, but the disks are sold in TiB, so the true figure is about 74.7 TiB.

Nothing is missing. 82.1 TB is correct and the cluster filled because retention was never enforced.