RAID 5 requires at least three hard disk drives. Data is stripped across all disks, and a portion of each disk is also used to store parity information for recovery if needed.
RAID 5RAID 5 implements disk striping for performance to all disks. Each disk also contains some parity information. Parity data is what allows the RAID to recover from a hard disk failure without data loss. RAID 5 arrays can consist of three or more disks, and support a hot spare disk.
The capacity of a RAID 5 array is equal to the smallest member hard disk drive capacity multiplied by the number of disk drives in the array minus 1 drive for parity. Example 1: (5) 200 GB drives in a RAID 5 equals 800 GB usable capacity. Here are the calculations: 5 x 200 GB = 1.0 TB of RAW capacity, minus 200 GB of parity (equal to one physical drive) leaves 800 GB of usable capacity. Example 2: (16) 500 GB drives in a RAID 5 equals 7.5 TB usable capacity. 16 x 500GB = 8.0 TB of RAW capacity, minus 500GB of overhead (one physical drive of parity) leaving 7.5 TB of usable capacity. Parity data is evenly distributed across all disks in the array but the total amount it consumes is equal to one member of the array. It is also important to note your file system overhead (such as NTFS) will add approximately 5% of overhead.
In the picture above, data is first striped between all disks, and then parity data is calculated and stored evenly between all disks as well. RAID 5 uses a complex algorithm whicih is sometimes referred to as XOR. Any RAID controller supporting an XOR ASIC (application-specific integrated circuit) will have improved RAID 5 performance.
If one disk fails in a RAID 5 array the status of the array is referred to as Degraded. RAID 5 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 disks #2 and #3 contain enough parity data so that when combined no data is lost. The RAID controller is able to make complex calculations and reconstruct the data that was stored on disk #1.
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