DBMS-Improving Reliability with RAID
For an array of n disks, the likelihood of failure is n times as much as that for one disk. Hence, if the MTTF (Mean Time To Failure) of a disk drive is assumed to be 200,000 hours or about 22.8 years (typical times range up to 1 million hours), that of a bank of 100 disk drives becomes only 2000 hours or 83.3 days. Keeping a single copy of data in such an array of disks will cause a significant loss of reliability. An obvious solution is to employ redundancy of data so that disk failures can be tolerated. The disadvantages are many: additional I/O operations for write, extra computation to maintain redundancy and to do recovery from errors, and additional disk capacity to store redundant information.One technique for introducing redundancy is called mirroring or shadowing. Data is written redundantly to two identical physical disks that are treated as one logical disk. When data is read, it can be retrieved from the disk with shorter queuing, seek, and rotational delays. If a disk fails, the other disk is used until the first is repaired. Suppose the mean time to repair is 24 hours, then the mean time to data loss of a mirrored disk system using 100 disks with MTTF of 200,000 hours each is (200,000)2/(2 * 24) = 8.33 * 108 hours, which is 95,028 years. Disk mirroring also doubles the rate at which read requests are handled, since a read can go to either disk. The transfer rate of each read, however, remains the same as that for a single disk.
Another solution to the problem of reliability is to store extra information that is not normally needed but that can be used to reconstruct the lost information in case of disk failure. The incorporation of redundancy must consider two problems: (1) selecting a technique for computing the redundant information, and (2) selecting a method of distributing the redundant information across the disk array. The first problem is addressed by using error correcting codes involving parity bits, or specialized codes such as Hamming codes. Under the parity scheme, a redundant disk may be considered as having the sum of all the data in the other disks. When a disk fails, the missing information can be constructed by a process similar to subtraction.
For the second problem, the two major approaches are either to store the redundant information on a small number of disks or to distribute it uniformly across all disks. The latter results in better load balancing. The different levels of RAID choose a combination of these options to implement redundancy, and hence to improve reliability.
