Calculate usable capacity of a RAID 6 array accounting for dual-disk parity.
RAID 6 extends RAID 5 by adding a second independent parity block, reserving the equivalent of two disks for parity data distributed across all members. This allows any two disks to fail simultaneously without data loss, making it safer for large arrays where the probability of a second failure during rebuild is non-trivial. The minimum disk count is 4.
RAID 6 usable capacity
usable = (n − 2) × disk_size
Storage efficiency
efficiency = (n − 2) / n × 100%
Use RAID 6 for arrays with large-capacity drives (4TB+) where rebuild times are long and a second failure during rebuild would be catastrophic, or for arrays with more than 8 disks where statistical likelihood of correlated failure increases.
RAID 6 has a higher write penalty than RAID 5 because each write operation must update two parity blocks instead of one — typically requiring 6 I/O operations per logical write vs 4 for RAID 5.