Few standard RAID levels are RAID 1 that follows mirroring, RAID 0 that follows striping and RAID 5 that follows parity summing. Following are the redundancy description of few RAID levels.
- If n is referred to number of drives used then RAID 1 offers redundancy of n-1 drives, where n is generally 2.
- If n-1 is the storage volume of RAID 5, then it permits one redundant drive.
- If RAID 1+0 contains two-mirrored RAID 0 arrays and each of them has five disks, then the minimum redundancy gets counted as 1 and maximum as 5. This is based on the consideration that the affected drives are of same array.
- If RAID 5+1 contains two-mirrored RAID 5 arrays and each of them has five disks, then the minimum redundancy comes out as 3 and maximum as 6.
- In RAID 5+5 array, three independent RAID 5 arrays are considered while each of them has three disks. This allows minimum redundancy as 3 and maximum as 5.
When a RAID array fails, data needs to be recovered using productive recovery measures that involve RAID specific utilities (which depend upon the RAID array or arrays being used) and RAID specific recovery practices (which are almost the same as the normal hard disk recovery). This is the specialized data recovery domain that deals with failed RAID hard drives and is specifically termed as RAID Recovery service. Under the service, failed hard drives are analyzed for cause of crash, repaired (physical repair is also done, if necessary) and data is extracted from the affected drive.
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