RAID (redundant array of independent disks)

Er · ‎16.03.2022

RAID (redundant array of independent disks)

Definition

A RAID system (redundant array of independent disks) is used to organize multiple physical mass storage devices (usually hard disk drives or solid-state drives) into a logical drive. The goals are to obtain either greater fail-safety or greater data throughput over a single physical storage medium.

The most commonly used RAID levels are:

RAID 0: Striping - acceleration without redundancy.

RAID 0 offers increased transfer rates by dividing the hard disks involved into contiguous blocks of the same size (striping). This requires at least two mass storage devices. The increase in data throughput is based on the fact that the necessary hard disk accesses can be processed in parallel to a greater extent.

Advantage: Data processing can be significantly increased by parallel writing and reading.

Disadvantage: If one of the storage media fails due to a defect, the data from the other is also no longer usable because the partial information from the defective medium is missing.

RAID 0 is therefore only recommended in applications where fail-safety is of little importance.

RAID 1: Mirroring

RAID 1 is a group of usually two mass storage devices. A RAID 1 stores the same data on all hard disks (mirroring) and thus offers full redundancy. However, the total capacity of the array is at most as large as the smallest hard disk involved.

Advantage: If one of the mirrored mass storage devices fails, the other can continue to supply all data.

Disadvantage: Mirroring reduces the actual storage capacity.

RAID 5: Performance + Parity, Block-Level Striping with Distributed Parity Information

RAID 5 implements striping with block-level distributed parity information. To calculate the parity, a logical group is formed by the data blocks of the hard disks involved in the RAID array, which are each at the same address. Of all the data blocks in a group, one data block contains the parity data, while the other data blocks contain user data. The user data of RAID 5 groups is distributed among all hard disks as in RAID 0. The parity information is also distributed. RAID 5 requires at least three mass storage devices.

Advantage: RAID 5 offers both increased data throughput when reading data and redundancy at relatively low cost.