RAID (Redundant Array of Independent Disks) combines multiple physical disk drives into a single logical unit to improve performance, fault tolerance, or both, using techniques like striping, mirroring, and parity.

What is RAID? Definition, examples and how it works

RAID (Redundant Array of Independent Disks) is a data storage virtualization technology that aggregates multiple physical disk drives into one or more logical units. It was originally defined in 1987 by David Patterson, Garth Gibson, and Randy Katz at the University of California, Berkeley. The goal is to improve performance through parallelism, increase reliability through redundancy, or achieve a balance of both.

RAID works by distributing data across drives using one or more of three fundamental techniques: striping (splitting data into blocks and writing them across multiple drives), mirroring (duplicating data identically on two or more drives), and parity (computing error-correcting information that can reconstruct lost data). Common levels include RAID 0 (striping, no redundancy), RAID 1 (mirroring), RAID 5 (striping with distributed parity, requiring at least three drives), RAID 6 (striping with dual distributed parity, tolerating two drive failures), and RAID 10 (a nested level combining RAID 1 mirrors striped as RAID 0). Each level offers different trade-offs in capacity efficiency, read/write performance, and fault tolerance.

RAID is implemented either in hardware (via a dedicated RAID controller with its own processor and cache) or in software (using the operating system or a volume manager like Linux mdadm or Windows Storage Spaces). It is a foundational building block in enterprise storage arrays, network-attached storage (NAS) appliances, and many server configurations. While RAID protects against single or multiple drive failures depending on the level, it is not a substitute for backups, as it does not guard against data corruption, accidental deletion, or site-wide disasters.

Key facts

RAID 0 offers maximum performance and capacity but zero fault tolerance.
RAID 1 mirrors data across two or more drives, providing redundancy with 50% capacity overhead.
RAID 5 uses distributed parity to tolerate one drive failure with at least three drives.
RAID 6 uses dual parity to tolerate up to two simultaneous drive failures.
RAID 10 combines mirroring and striping, requiring an even number of drives (minimum four).

How it works in practice

A database server hosting a high-transaction workload might use RAID 10 across four 1 TB SSDs. This gives 2 TB usable capacity, fast reads and writes from striping, and fault tolerance because each drive is mirrored. If one drive fails, the system continues operating from its mirror while the failed drive is hot-swapped and rebuilt.

Related terms

JBOD striping mirroring parity RAID controller NAS SAN

References

More in Storage

Block Storage

Block storage is a storage architecture that presents raw fixed-size chunks (blocks) to a computer, which the operating system partitions and formats before use, commonly used for virtual machine disks and database volumes.

Cold Storage

Cold storage is a low-cost, slow-retrieval data tier for infrequently accessed information, such as backups, archives, or compliance records, with retrieval times ranging from minutes to hours.

Erasure Coding

Erasure coding is a data protection method that splits data into fragments (shards) and adds parity, allowing reconstruction from a subset of fragments; it is commonly used in cloud object storage to tolerate failures efficiently.

Fibre Channel

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File Storage

File storage exposes data as a hierarchical filesystem over network protocols like NFS and SMB, commonly delivered via network-attached storage (NAS) appliances for shared access.

Hot Storage

Hot storage is the default high-performance tier in object storage systems, optimized for low-latency access to frequently retrieved data.

iSCSI

iSCSI is a storage protocol that encapsulates SCSI block-level commands over TCP/IP, enabling servers to access remote disks as if they were locally attached.

NFS

NFS (Network File System) is a distributed file system protocol originally developed by Sun Microsystems that allows a client to access files over a network as if they were local. It is widely used in Unix and Linux environments and runs over IP networks.

Object Storage

Object storage is a flat data storage architecture that manages data as immutable objects, each with a unique identifier, metadata, and API access over HTTP. It is the model behind Amazon S3 and similar cloud storage services.

RAID 0

A data storage method that splits data across two or more disks in stripes to improve read and write performance, but provides no fault tolerance.