What is RAID 5?
RAID 5 is a block-level striping configuration with distributed parity that can survive a single disk failure while using less total capacity for redundancy than mirroring.
RAID 5 is a disk array technology that combines striping with distributed parity to provide fault tolerance with lower overhead than RAID 1 mirroring. Data blocks are striped across all member disks, and a parity block computed from the data blocks is also written to a disk. Unlike RAID 4, which uses a dedicated parity disk, RAID 5 rotates the parity block across all disks. This spreads the write load evenly and avoids a single parity bottleneck.
When a single disk fails, reading its data blocks is done by reconstructing them on the fly using the remaining data and parity blocks on the other disks. Once the failed disk is replaced, the array rebuilds its contents. RAID 5 can tolerate exactly one disk failure; a second failure at any time before the rebuild completes causes permanent data loss. The usable capacity of an N-disk RAID 5 array is (N-1) times the size of the smallest disk. Write performance is slightly reduced compared to RAID 0 because parity must be calculated for every write. Sequential read performance is excellent because all spindles are used in parallel.
RAID 5 sits between RAID 1 (mirroring, 50 percent capacity overhead) and RAID 6 (dual parity, tolerates two failures) in the standard RAID taxonomy. It is commonly used in file servers, media storage, and general-purpose NAS appliances where capacity and single-fault tolerance are required. However, with large modern disk sizes and long rebuild times, RAID 5 exposes data to an increased risk of an unrecoverable read error or second failure during rebuild. Many enterprise storage systems now recommend RAID 6 for arrays with large-capacity drives.
Key facts
- Distributes parity blocks across all disks, not a dedicated parity disk
- Tolerates exactly one disk failure without data loss
- Usable capacity is (N-1) times the size of the smallest disk in the array
- Sequential read performance approaches N times a single disk's rate
- Random write performance suffers from read-modify-write penalty for parity updates
How it works in practice
Related terms
References
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