The "mean time to data loss" (MTTDL) gives the average time before a loss of data happens in a given array. Mean time to data loss of a given RAID may be higher or lower than that of its hard disks. This depends on the type of RAID used.
Arrays that have redundancy can recover from some failures. The "mean time to recovery" shows how long it takes until a failed array is back to its normal state. This adds both the time to replace a failed disk mechanism as well as time to re-build the array (i.e. to replicate data for redundancy).
The "unrecoverable bit error rate" (UBE) tells how long a disk drive will be unable to recover data after using cyclic redundancy check (CRC) codes and multiple retries.
There are also certain problems with the ideas or the technology behind RAID:
Certain RAID levels allow to extend the array by simply adding hard disks, at a later time. Information such as parity blocks is often scattered on several disks. Adding a disk to the array means that a reorganisation becomes necessary. Such a reorganisation is like a re-build of the array, it can take a long time. When this is done, the additional space may not be available yet, because both the file system on the array, and the operating system need to be told about it. Some file systems do not support to be grown after they have been created. In such a case, all the data needs to be backed up, the array needs be re-created with the new layout, and the data needs to be restored onto it.
Another option to add storage is to create a new array, and to let a logical volume manager handle the situation. This allows to grow almost any RAID system, even RAID1 (which by itself is limited to two disks).
The error correction mechanism in RAID assumes that failures of drives are independent. It is possible to calculate how often a piece of equipment can fail and to arrange the array to make data loss very improbable.
In practice, however, the drives were often bought together. They have roughly the same age, and have been used similarly (called "wear"). Many drives fail because of mechanical problems. The older a drive is, the more worn are its mechanical parts. Mechanical parts that are old are more likely to fail than those that are younger. This means that drive failures are no longer statistically independent. In practice, there is a chance that a second disk will also fail before the first has been recovered. This means that data loss can occur at significant rates, in practice.
Another problem that also occurs with RAID systems is that applications expect what is called "Atomicity": Either all of the data is written, or none is. Writing the data is known as a "transaction".
In RAID arrays, the new data is usually written in the place where the old data was. This has become known as "update in-place". Jim Gray, a database researcher wrote a paper in 1981 where he described this problem.
Very few storage systems allow "atomic write semantics". When an object is written to disk, a RAID storage device will usually be writing all copies of the object in parallel. Very often, there is only one processor responsible for writing the data. In such a case, the writes of data to the different drives will overlap. This is known as "overlapped write" or "staggered write". An error that occurs during the process of writing may therefore leave the redundant copies in different states. What is worse, it may leave the copies in neither the old nor the new state. Logging relies on the original data being either in the old or the new state, though. This permits backing out the logical change, but few storage systems provide an atomic write semantic on a RAID disk.
Using a battery-backed write cache can solve this problem, but only in a power failure scenario.
Transactional support is not present in all hardware RAID controllers. Therefore, many operating systems include it to protect against data loss during an interrupted write. Novell Netware, starting with version 3.x, included a transaction tracking system. Microsoft introduced transaction tracking via the journaling feature in NTFS. NetApp WAFL file system solves it by never updating the data in place, as does ZFS.
Some sectors on a hard disk may have become unreadable because of a mistake. Some RAID implementations can deal with this situation by moving the data elsewhere and marking the sector on the disk as bad. This happens at about 1 bit in 10 in enterprise-class disk drives, and 1 bit in 10 in ordinary disk drives. Disk capacities are steadily increasing. This may mean that sometimes, a RAID cannot be rebuilt, because such an error is found when the array is rebuilt after a disk failure. Certain technologies such as RAID 6 try to address this issue, but they suffer from a very high write penalty, in other words writing data becomes very slow.
The disk system can acknowledge the write operation as soon as the data is in the cache. It does not need to wait until the data has been physically written. However, any power outage can then mean a significant data loss of any data queued in such a cache.
With hardware RAID, a battery can be used to protect this cache. This often solves the problem. When the power fails, the controller can finish writing the cache when the power is back. This solution can still fail, though: the battery may have worn out, the power may have been off for too long, the disks could be moved to another controller, the controller itself could fail. Certain systems can do periodic battery checks, but these use the battery itself, and leave it in a state where it is not fully charged.
The disk formats on different RAID controllers are not necessarily compatible. Therefore, it may not be possible to read a RAID array on different hardware. Consequently, a non-disk hardware failure may require using identical hardware, or a backup, to recover the data.
This guide was taken from a thread in a RAID-related forum. This was done to help point out the advantages and disadvantages of choosing RAID. It is directed at people who want to choose RAID for either increases in performance or redundancy. It contains links to other threads in its forum containing user-generated anecdotal reviews of their RAID experiences.
The RAID levels used most often are RAID 0, RAID 1, and RAID 5. Suppose there is a 3 disk setup, with 3 identical disks of 1 TB each, and the probability of failure of a drive for a given timespan is 1%.
Leonid Brezhnev
Leonid Illich Brezhnev (19 December 1906 - 10 November 1982) was a leader of the Soviet Union. Born in 1906, Brezhnev was First Secretary of the Communist Party of the Soviet Union, effectively the dictator of the Soviet Union from 1964 until his death in 1982. During the Second World War, Brezhnev was a Political Commissar, supervising various military units. There, he became a friend of Nikita Khrushchev.
Unlike Khrushchev, the leader of the Soviet Union before him, Brezhnev did not agree with the process of destalinization, and did not support reform. Under Brezhnev, the Soviet economy mostly consisted of military spending. Because the country's resources were going to the armed forces, as they had under Joseph Stalin in wartime, consumer wants were ignored. The standard of living began to fall.
Brezhnev died of a heart attack on November 10, 1982.
Approximation
An approximation is a version of a piece of information that does not describe it exactly, but is close enough to be used. An approximation may be used either when the exact piece of information is not known, or when it's too long or complicated and people need something simpler.
For example, if you wanted to add 2.0002 and 2.0003, you can add 2 and 2, and it would be an approximation.
If you are left with a balance of Dollar 78.23 you might want to say you are left with 80 dollars. In mathematics, this is written using the formula_1 symbol (e.g., formula_2).