What Is RAID? Redundant Array of Independent Disks Explained

RAID relies on three core techniques, combined differently at each level. Striping splits data into blocks and writes them across multiple drives at once, which increases throughput because several disks read and write in parallel — but striping alone adds no protection. Mirroring writes an identical copy of data to two or more drives, so a full copy survives if one fails. Parity stores mathematically derived information — single parity is computed with an XOR operation — that lets the array reconstruct the data from a failed drive using the surviving drives and the parity blocks.

A RAID array is presented to the operating system as one volume, so applications see a single disk regardless of how many physical drives sit behind it. The controller — whether a dedicated hardware card or a software layer — handles the distribution of data, the parity math, and the reconstruction of a failed drive. Different levels place these functions differently, which is what produces the trade-offs between redundancy, speed, and usable capacity.

RAID 0 stripes data across drives for maximum performance and full capacity but has zero redundancy — one drive failure loses the entire array, so it suits scratch or temporary data only. RAID 1 mirrors data across two (or more) drives; it survives a drive failure and reads quickly, but usable capacity is halved. RAID 5 stripes data with distributed parity across three or more drives, tolerating one drive failure while sacrificing only a single drive's worth of capacity to parity — a common balance of protection, performance, and efficiency.

RAID 6 extends RAID 5 with a second, independent parity block, so it survives two simultaneous drive failures at the cost of two drives' worth of capacity; this matters as drive counts and capacities grow and rebuild windows lengthen. RAID 10 (a nested level, 1+0) mirrors pairs of drives and then stripes across those mirrors, delivering strong performance and fast rebuilds while tolerating a failure in each mirror set — but usable capacity is again roughly half the raw total. Higher redundancy and performance generally cost usable capacity, and no single level is best for every workload.

When a drive fails in a redundant array, the array runs in a degraded state — still accessible, but with reduced or no remaining fault tolerance. After the failed drive is replaced, the controller rebuilds it by reconstructing its data from the mirror copy or from parity and the surviving drives. During this window the array is more vulnerable: in RAID 5 a second failure before the rebuild completes loses the array, which is a key reason RAID 6 exists for large, high-capacity arrays.

Rebuild time scales with drive capacity and the array's activity level, and large modern drives can take many hours to rebuild while the whole array reads under load. The rebuild also stresses the remaining drives, which is when latent faults sometimes surface. Practical mitigations include hot spares that begin rebuilding automatically, monitoring and alerting so failures are caught early, and choosing a redundancy level appropriate to drive size and how quickly failed drives can be replaced.

Hardware RAID uses a dedicated controller card with its own processor to manage the array, offloading parity calculations from the host CPU. Controllers often include battery- or flash-backed write cache, which improves write performance and protects in-flight data during a power loss. The trade-offs are cost, and dependence on the controller — recovering an array can require a compatible controller, so a spare or documented replacement path is worth planning for.

Software RAID is implemented by the operating system or a filesystem/volume manager and runs on the host CPU, avoiding dedicated hardware cost and controller lock-in. Modern CPUs handle parity easily for most workloads, and software or filesystem-integrated approaches can add data-integrity features such as checksumming. The main considerations are host CPU and memory use under heavy load and, for boot volumes, ensuring the platform can start from a software array. Both approaches are valid; the right choice depends on workload, budget, and operational preferences.