RAID Configurations

Failure rate
The mean time to failure (MTTF) of a given RAID may be lower or higher than those of its constituent hard drives, depending on what type of RAID is employed.
Mean time to data loss (MTTDL)
In this context, the average time before a loss of data in a given array.

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 disk mechanism as well as time to re-build the array (i.e. to replicate data for redundancy).

Unrecoverable bit error rate (UBE)
This is the rate at which a disk drive will be unable to recover data after application of cyclic redundancy check (CRC) codes and multiple retries. This failure will present as a sector read failure. Some RAID implementations protect against this failure mode by remapping the bad sector, using the redundant data to retrieve a good copy of the data, and rewriting that good data to the newly mapped replacement sector. The UBE rate is typically specified at 1 bit in 1015 for enterprise class disk drives (SCSI, FC, SAS) , and 1 bit in 1014 for desktop class disk drives (IDE, ATA, SATA). Increasing disk capacities and large RAID 5 redundancy groups have led to an increasing inability to successfully rebuild a RAID group after a disk failure because an unrecoverable sector is found on the remaining disks. Double protection schemes such as RAID 6 are attempting to address this issue, but suffer from a very high write penalty.

Atomic Write Failure
Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, etc. This is a little understood and rarely mentioned failure mode for redundant storage systems. Database researcher Jim Gray wrote "Update in Place is a Poison Apple" during the early days of relational database commercialization. However, this warning largely went unheeded and fell by the wayside upon the advent of RAID, which many software engineers mistook as solving all data storage integrity and reliability problems. Many software programs update a storage object "in-place"; that is, they write a new version of the object on to the same disk addresses as the old version of the object. While the software may also log some delta information elsewhere, it expects the storage to present "atomic write semantics", meaning that the write of the data either occurred in its entirety or did not occur at all. However, very few storage systems provide support for atomic writes, and even fewer specify their rate of failure in providing this semantic. Note that during the act of writing an object, a RAID storage device will usually be writing all redundant copies of the object in parallel. Hence an error that occurs during the process of writing may leave the redundant copies in different states, and furthermore may leave the copies in neither the old nor the new state. The little known failure mode is that delta logging relies on the original data being either in the old or the new state so as to enable backing out the logical change, yet few storage systems provide an atomic write semantic on a raid disc.

Jagath Krishnakumar