How the RAID Storage Capacity & Fault Tolerance Calculator Works — The Formula Explained
The RAID Storage Capacity & Fault Tolerance Calculator demystifies one of the most critical aspects of data storage: how much usable space you actually get and how resilient your data is to drive failures. It's not a single formula, but rather a set of calculations based on the specific RAID level chosen, each with its own methodology for distributing data and parity (or mirroring).
At its core, the calculator takes two primary inputs:
- Number of Drives (N): The total count of physical hard disk drives (HDDs) or solid-state drives (SSDs) in your array.
- Size of Each Drive (D): The raw capacity of each individual drive, typically measured in terabytes (TB) or gigabytes (GB). For simplicity, all drives in a RAID array should ideally be of the same capacity. If different sizes are used, the effective capacity of each drive will be limited to that of the smallest drive in the array.
Let's break down the capacity and fault tolerance logic for common RAID levels:
- RAID 0 (Striping):
- Usable Capacity: N × D
- Fault Tolerance: 0 drive failures. If any single drive fails, all data is lost.
- Explanation: Data is striped across all drives without any redundancy. This maximizes capacity and often performance but offers no data protection.
- RAID 1 (Mirroring):
- Usable Capacity: D (for an array of N drives, where N is even, the capacity is D. For two drives, it's 1 drive's capacity. For 4 drives, it's 2 drives' capacity etc. The calculator assumes a single mirrored pair for simplicity, effectively D). For N drives, it's (N/2) * D.
- Fault Tolerance: 1 drive failure per mirrored pair. In a 2-drive RAID 1, it tolerates 1 failure. In a 4-drive RAID 1 (two mirrored pairs), it tolerates 2 failures (one from each pair).
- Explanation: Data is duplicated (mirrored) across two drives. This provides excellent redundancy and read performance but sacrifices 50% of raw capacity. For an array with N drives, it creates N/2 mirrored pairs.
- RAID 5 (Striping with Single Parity):
- Usable Capacity: (N - 1) × D
- Fault Tolerance: 1 drive failure.
- Explanation: Data is striped across all drives, and a single parity block is distributed across all drives. This provides good capacity utilization and fault tolerance for a single drive failure. Requires a minimum of 3 drives.
- RAID 6 (Striping with Dual Parity):
- Usable Capacity: (N - 2) × D
- Fault Tolerance: 2 drive failures.
- Explanation: Similar to RAID 5, but with two independent parity blocks distributed across all drives. This allows for two simultaneous drive failures, offering superior resilience, especially for large arrays. Requires a minimum of 4 drives.
- RAID 10 (Striping of Mirrors / RAID 1+0