"Eleven nines of durability" is the number cloud storage quietly leans on — 99.999999999% odds your object survives the year. It comes from a short sum: keep enough copies, and you only lose data if every copy dies inside the window it takes to rebuild a lost one. Feed in the copies, how often a drive fails, and how fast you repair, and read off the nines.
That's 99.9999999% odds a given object survives the year — roughly 1 loss in 2.8 billion object-years.
With 1 billion objects stored, this durability means you'd expect to lose one about every 3 years — comfortably rare.
| Copies | Durability |
|---|---|
| 1 copy | 1 nines 98% |
| 2 copys | 5 nines 99.999% |
| 3 copysnow | 9 nines 99.9999999% |
| 4 copys | 13 nines 99.99999999999% |
| 5 copys | 16 nines 99.99999999999999% |
| 6 copys | 20 nines 99.999999999999999999% |
Each copy multiplies the odds the rest are all gone before a rebuild finishes, so durability climbs in big jumps — every replica adds several nines at once. The same lever works the other way: halve the rebuild time and you shrink the window for a cascade, buying nines without buying disks.