RAID 4: Block-level striping with dedicated parity.

Each block is written onto a data disk. Parity for each blocks is generated during writes and recorded on the parity disk and checked on reads.

Capacity: (Size of smallest disk) * (Number of disks - 1)

Read Performance is very good.(Better than Raid 5)
Fault tolerant, can loose one or more disks with little to no impact.

Expensive controller needed to support RAID 4.
Write performance is fair.
Long rebuild times on failed disks.

RAID 4 is not as commonly used as RAID 3 and RAID 5, because it is a comprimise between the two that doesn't have a well-defined target market.

Network Appliance {, a creator of CIFS/NFS capable network atached storage NAS solutions is a primary user of RAID 4.
Network Appliance is also known for their Write Anywhere File Layout WAFL®.

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