RAID (Redundant Array of Independent Disks) combines multiple hard drives to improve performance, reliability, or both. Different RAID levels use different strategies: mirroring (copying data), striping (distributing data), and parity (error correction). Understanding RAID capacity, speed, and redundancy trade-offs is essential for system administrators and anyone managing significant data.
Common RAID Levels
RAID 0 (Striping):
- Data split across drives, no redundancy
- Usable capacity: n ร drive size (100% utilization)
- Speed: fastest
- Reliability: worst (any drive failure loses all data)
RAID 1 (Mirroring):
- Identical copy on second drive
- Usable capacity: n ร drive size / 2 (50% utilization)
- Speed: fast reads, slower writes
- Reliability: excellent (one drive can fail)
RAID 5 (Striping with Parity):
- Data and parity spread across 3+ drives
- Usable capacity: (n - 1) ร drive size
- Speed: moderate
- Reliability: good (any one drive can fail)
RAID 6 (Dual Parity):
- Like RAID 5, but two-drive redundancy
- Usable capacity: (n - 2) ร drive size
- Speed: moderate
- Reliability: very good (any two drives can fail)
RAID 10 (Mirrored Stripe):
- Combination of RAID 1 and RAID 0
- Requires minimum 4 drives
- Usable capacity: 50%
- Speed: very fast
- Reliability: very good
Worked Example
Building storage with 4 ร 4TB drives:
| RAID Level | Usable Capacity | Fault Tolerance |
|---|---|---|
| RAID 0 | 16 TB | None |
| RAID 1 | 4 TB | 1 drive |
| RAID 5 | 12 TB | 1 drive |
| RAID 6 | 8 TB | 2 drives |
| RAID 10 | 8 TB | 1 drive per mirror |
RAID 5 balances capacity (12 TB) and reliability. RAID 6 sacrifices 4 TB but protects against two simultaneous failures.
Rebuild Time Considerations
After a drive fails, the array must rebuild by reconstructing lost data from parity or the mirror. Rebuild time = (Array Capacity) / (Rebuild Speed). Large RAID arrays take days to rebuild, during which another drive failure causes complete data loss. RAID 6 is recommended for large arrays.
Tips
RAID is not backup. It protects against hardware failure, not user error, malware, or data corruption. Always maintain an independent backup. Also remember that RAID doesn't improve performance indefinitely โ controller and bus speed become bottlenecks. Match RAID level to your reliability and capacity needs.
Use our Storage RAID Calculator to compare usable capacity and fault tolerance across RAID configurations.