- Suppose that the mean time to failure of a single disk is 100000 hours. Then the mean time to failure of some disk in an array of 100 disks will be 100000/100 = 1000 hours, or 41.66 days, which is not long at all.
- If we store only one copy of the data, then each disk failure will result in loss of a significant amount of data-and such a high rate of data loss is unacceptable.
- The solution to the problem of reliability is to introduce redundancy; we store extra information that is not normally needed but that can be used in the event of failure of a disk to rebuild the lost information.
- Thus, even if a disk fails, data are not lost.
- The simplest (but most expensive) approach to introducing redundancy is to duplicate every disk. This technique is called mirroring.