RAID 3 requires at least three hard disk drives. Data is striped across all disks but one, and that one is a dedicated parity drive containing parity information for data recovery if needed.
RAID 3RAID 3 implements disk striping for performance to all disks but one in the array. The remaining disk in the array is used to hold all parity data. Parity data is what allows the RAID to recover from a hard disk failure without data loss. RAID 3 arrays can consist of three or more disks, and support a hot spare disk.
The capacity of a RAID 3 array is equal to the smallest member hard disk drive capacity multiplied by the number of disk drives in the array, minus one disk for parity. Example 1: (6) 200 GB drives in a RAID 3 equals 1 TB capacity (6 x 200 GB = 1.2 TB, minus 200 GB for parity). Example 2: (1) 73 GB drive and (2) 146 GB drives in a RAID 3 equals 146 GB capacity (3 x 73 GB = 219 GB, minus 73 GB for parity).
In the picture above, data is first striped between hard disk #1 and #2, and then parity data is stored on hard disk #3. RAID 3 is rarely used, most applications now use RAID 5 instead.
If one disk fails in a RAID 3 array the status of the array is referred to as Degraded. RAID 3 allows for one disk failure without the loss of any data. When an array is degraded, the performance and efficiency of the array suffers, but the data on the array is still available. In the above example, if disk #1 failed, then disk #3 contains enough parity data so that when combined with disk #2 no data is lost. The RAID controller is able to make complex calculations and reconstruct the data that was stored on disk #1. If disk #3 had failed, then disk #1 and #2 still contain all the data, no parity calculation is required to recover it.
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