A RAID 5 array is one of the most popular choices for RAID arrays due to its excellent performance and fault tolerance. RAID 5, also called striped disks with parity, combines three or more disks in a way that protects data against loss of any one disk. The fault tolerance used in a RAID 5 array is achieved by generating a parity block for each data block that is written on a disk in the array. A series of blocks is known as a stripe and each time a new block is written on the stripe the parity block is recalculated and rewritten. The parity blocks are known as "distributed parity blocks" because they are staggered on different stripes.
Parity blocks are only read with there is a CRC error and not when the data is read since that would decrease performance. If an error occurs the parity blocks and data blocks from the other disks are used to reconstruct the data. The disadvantage of a RAID 5 array is that since only one parity block is written to each stripe so if two hard drives fail total data loss will occur. However if you are in the habit of backing up your files frequently, you may still want to consider a RAID 5 array for the benefit of increased performance.
Three matching hard drives are necessary to setup a RAID 5 array. The storage capacity of the array is reduced by one hard disk.
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