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)

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

Disadvantages:
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 {www.networkappliance.com), 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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