1 Given a RAID 0 array of 2 drives with a 5 chance of failur

1) Given a RAID 0 array of 2 drives with a 5% chance of failure within a specified time frame, what is the failure probability of the array within that time frame?

Solution

Consider a RAID 1 with two identical models of a disk drive with a 5% probability that the disk would fail within three years. Provided that the failures are statistically independent, then the probability of both disks failing during the three year lifetime is 0.25%. Thus, the probability of losing all data is 0.25% over a three year period if nothing is done to the array. If the first disk fails and is never replaced, then there is a 5% chance the data will be lost. If only one of the disks fails, no data would be lost. As long as a failed disk is replaced before the second disk fails, the data is safe.

However, since two identical disks are used and since their usage patterns are also identical, their failures cannot be assumed to be independent. Thus, the probability of losing all data, if the first failed disk is not replaced, may be considerably higher than 5%.

As a practical matter, in a well-managed system the above is irrelevant because the failed hard drive will not be ignored but will be replaced. The reliability of the overall system is determined by the probability the remaining drive will continue to operate through the repair period, that is the total time it takes to detect a failure, replace the failed hard drive, and for that drive to be rebuilt. If, for example, it takes one hour to replace the failed drive, the overall system reliability is defined by the probability the remaining drive will operate for one hour without failure.