RAID Calculator

Understanding RAID Arrays

RAID stands for Redundant Array of Independent Disks—a technique that combines multiple drives into a single logical unit to improve either performance, fault tolerance, or both. In the early era of computing, expensive mainframe drives were highly reliable but catastrophically expensive when they failed. The modern solution reversed the economics: use many affordable, commodity drives working in parallel to achieve reliability and speed simultaneously.

Different RAID levels distribute data across drives using two key strategies:

• Striping—data is split across multiple drives, allowing simultaneous reads and writes for speed.
• Mirroring—identical copies of data exist on separate drives, so one failure doesn't cause data loss.

Every RAID level makes a deliberate trade-off: more fault tolerance means fewer usable drives; maximum speed sacrifices redundancy; lower cost-per-terabyte comes with reduced protection.

Core RAID Levels: 0, 1, 5, and 6

RAID 0 (Striping) distributes data in blocks across all drives, enabling simultaneous access. Read and write speeds scale linearly with disk count—four drives yield 4× performance—but any single drive failure destroys the entire array. No fault tolerance. Best for non-critical, high-speed workloads.

RAID 1 (Mirroring) copies every block to a second drive. One drive can fail without any data loss. Read performance improves slightly since requests can be load-balanced; write performance remains limited to a single drive's speed. Usable capacity is 50% of total. Expensive per gigabyte but extremely reliable.

RAID 5 (Striping with Parity) spreads data and a calculated parity block across three or more drives. Any single drive failure can be reconstructed from parity and the remaining data. Read performance approaches the sum of all drives minus one; write performance is constrained by parity calculation. More efficient than RAID 1—usable capacity is (n − 1) × disk size for n drives.

RAID 6 (Double Parity) extends RAID 5 by adding two independent parity blocks, tolerating any two simultaneous drive failures. Write penalty is higher due to double parity computation, but fault tolerance during rebuild operations is superior. Usable capacity is (n − 2) × disk size.

Nested and Extended Levels: 10, 50, 60, 1E, 5E, 5EE

RAID 10 (1+0) mirrors pairs of drives, then stripes those pairs together. Requires an even number of disks and allows one failure per mirrored set—up to two total failures if they occur in different pairs. Usable capacity is always 50%, but rebuild times are fast because data reads directly from the surviving mirror without parity recalculation.

RAID 50 and 60 nest RAID 5 or RAID 6 sub-arrays beneath a RAID 0 stripe layer. This improves rebuild times: a failed drive is rebuilt from its own parity group rather than the entire array. RAID 50 tolerates one failure per sub-array; RAID 60 tolerates two per sub-array. Both scale capacity and fault tolerance better than single RAID 5 or 6 arrays on large drive counts.

RAID 1E pairs striping and mirroring across an odd number of drives. Each stripe is mirrored to the next disk. One failure is tolerated, and usable capacity is 50%. Useful when you have an odd number of drives but want better performance than simple RAID 1.

RAID 5E and 5EE add a hot spare area to RAID 5, allowing immediate rebuild without manual drive swapping. RAID 5E stores spare space at the end of each drive; RAID 5EE distributes it throughout. Both require a minimum of four drives.

Capacity, Speed, and Fault Tolerance Formulas

The calculator derives usable capacity, performance multipliers, and fault tolerance limits for each RAID level. Below are the core equations:

• Number of Disks — Total drives in the RAID array.
• Disk Size — Capacity of each individual drive (in GB or TB).
• Usable Capacity — Raw total capacity minus overhead for redundancy and parity.
• Speed Gain — Theoretical maximum read or write performance multiplier versus a single disk.
• Fault Tolerance — Maximum number of simultaneous disk failures the array can survive without data loss.

Practical Considerations When Choosing RAID

Real-world RAID deployments introduce constraints and pitfalls beyond raw metrics.

1. Rebuild Time and URE Risk — Modern multi-terabyte drives have an unrecoverable read error (URE) rate of roughly 1 in 10¹⁵ bits. During a RAID 5 rebuild of a 4 TB failed drive, you read 4 TB × 8 trillion bits from other drives; the probability of hitting a URE approaches 30%. RAID 6 or nested arrays (50, 60) mitigate this by reducing parity read scope and rebuild speed.
2. Write Penalty and Small Random I/O — RAID 5 and 6 incur a significant write penalty: each logical write requires reading old data, reading old parity, computing new parity, and writing data plus new parity—often 4–6 drive I/Os per single logical write. RAID 10 and RAID 0 write more efficiently. For databases or transactional workloads, this penalty compounds and can severely impact throughput.
3. Capacity Utilization vs. Cost — RAID 1 and RAID 10 waste 50% of raw capacity but offer fast, simple recovery. RAID 5 loses only one drive's worth, making it attractive for bulk storage—but at the cost of rebuild time. RAID 6 and nested levels (50, 60) improve both, but drive count and complexity increase. Calculate cost-per-usable-terabyte, not just total capacity.
4. Controller and Cache Requirements — Enterprise RAID controllers feature battery-backed write caches to preserve data integrity during power loss and to accelerate parity writes. Software RAID or budget controllers without caches are vulnerable to corruption if a crash occurs mid-parity-write. Always verify your controller's cache strategy before committing critical data.