RAID - Reliability Terms

Reliability Terms

Failure rate

Two different kinds of failure rates are applicable to RAID systems. Logical failure is defined as the loss of a single drive and its rate is equal to the sum of individual drives' failure rates. System failure is defined as loss of data and its rate will depend on the type of RAID. For RAID 0 this is equal to the logical failure rate, as there is no redundancy. For other types of RAID, it will be less than the logical failure rate, potentially very small, and its exact value will depend on the type of RAID, the number of drives employed, the vigilance and alacrity of its human administrators, and chance (improbable events do occur, though infrequently).

Mean time to data loss (MTTDL)

In this context, the average time before a loss of data in a given array. Mean time to data loss of a given RAID may be higher or lower than that of its constituent hard drives, depending upon what type of RAID is employed. The referenced report assumes times to data loss are exponentially distributed, so that 63.2% of all data loss will occur between time 0 and the MTTDL.

Mean time to recovery (MTTR)

In arrays that include redundancy for reliability, this is the time following a failure to restore an array to its normal failure-tolerant mode of operation. This includes time to replace a failed drive mechanism and time to re-build the array (to replicate data for redundancy).

Unrecoverable bit error rate (UBE)

This is the rate at which a drive will be unable to recover data after application of cyclic redundancy check (CRC) codes and multiple retries.

Write cache reliability

Some RAID systems use RAM write cache to increase performance. A power failure can result in data loss unless this sort of drive buffer has a supplementary battery to ensure that the buffer has time to write from RAM to secondary storage before the drive powers down.

Atomic write failure

Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, non-transactional, etc.