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RAID was originally an acronym for Redundant Array of Inexpensive Disks. Each RAID level describes the method in which data is distributed across a number of disk drives, known as an array. It must be noted however, that these are not true "levels", as the higher levels don't contain all the functions of the lower levels, and RAID 5 is not necessarily better than RAID 3. The birth of drive arrays effectively started with a paper published by Berkeley University (CA), this paper classified the different architectures as RAID levels. The driving force was to provide minicomputers with high performance drive subsystems using the (then) newly available low cost 5.25" Winchesters rather than the very expensive mass storage magnetic drives used by mainframes. The concept was to provide the required capacity using multiple 5.25" drives connected as an array; in effect the drives were operated in parallel to increase their performance. The word Inexpensive in RAID's original definition is now somewhat of a misnomer. The drives used in a RAID solution tend to be high quality, fast, SCSI devices which are by no means the cheapest drives available for PCs. Consequently, the definition of RAID is undergoing a change from Redundant Array of Inexpensive Disks to Redundant Array of Independent Disks. RAID: Redundant Array of Independent Disks The original Berkeley paper described five RAID levels. Of these only RAID 1, 3 and 5 are now considered practical solutions. RAID 2 and 4 have been superseded by RAID 3 and 5 respectively as they offer similar or superior performance at no additional cost. There are other levels of RAID, but of these, only RAID 0 is accepted as an industry standard. Note: For the purpose of this document, the diagrams illustrate a RAID solution using either two or four disk drives - this is not a mandatory requirement. Each RAID solution should be tailored to the customer's application using the appropriate number of drives. The table below give the raid levels supported by the Mylex RAID controller.
In recent years, the use of computers to store and process information has risen dramatically. Literally every major business uses the computer as a tool to compete in industry. The necessity to use computers to compete has driven the need for higher performance systems. Rapid access to information is critical. Equally as important is the safety and availability of information stored on computer systems. Redundancy: The key to designing a high availability system is component redundancy. If a RAID system goes down because of a single point of failure, it is of limited use in a high availability environment. Performance: High data availability is very important, but not at the expense of performance. RAID system performance is largely a factor of the RAID controllers design. In particular, a RAID controller should offer read cache and the ability to tune a I/O chunk size for different applications. A large read cache is very important to a RAID system’s overall performance. The Mean Time Between Failure (MTBF) of the array will be equal to the MTBF of an individual drive, divided by the number of drives in the array, because of this the MTBF of an array of drives would be too low for many applications. However, disk arrays can be made fault-tolerant by redundantly storing information in various ways. There are several types of array architectures, RAID-0 through RAID-5, as defined in the Berkeley paper, each providing disk fault-tolerance and each offering different trade-offs in features and performance. These may be achieved by several methods. There are three primary ways of implementing an array:
Primarily used with entry level servers. These type of arrays rely on the use of a standard host adapter and execute all commands relevant to the operation of the array in the host server cpu. This can slow down system performance by increasing host PCI bus traffic, CPU utilisation and CPU interrupts. Some network operating systems such as NT and Novell include the software for such implementation.
This system uses a dedicated array controller that plugs into the host systems bus slot. All of the relevant array commands are performed by secondary processors on the controller. In addition to offering the fault-tolerance benefits of RAID, bus based array controllers perform connectivity functions that are similar to standard host adapters. By residing on the host bus they provide the highest performance of all array types. Extra memory and battery back-up modules may be fitted to these controllers.
SCSI to SCSI array uses a standard host adapter to connect to an external array controller. The external controller has its own processor and memory that provides high performance.
RAID-0 does not provide any data protection. Instead, its purpose results from the need for fast data availability. The technique used in a RAID-0 solution is called stripping. Stripping is performed at a block level in RAID-0, but without data redundancy. If any drive should fail, the entire array will fail and data will be lost up to the last system backup. RAID 0 was not described by the Berkeley team but due to its wide availability has since been endorsed by the RAID Advisory Board (RAB). RAB is an association of suppliers and consumers of RAID related products and other organisations with an interest in RAID technology set up to standardise RAID related terminology throughout the industry. RAID 0, also known as disk striping, works by segmenting a user file into 'data blocks' and then writing the blocks sequentially across the drives in the array. Disk striping offers excellent performance by transferring data using several disks at once. However, in this configuration only one copy of the data is stored, and therefore no fault tolerance is achieved. In fact this method is even more open to data loss; when four drives make up a single logical drive, that logical drive is four times more likely to fail than a single physical disk. Strictly speaking RAID level 0 is not a RAID at all as there is no redundancy. RAID 0 can be implemented on any NetWare or Windows NT system with more than 1 hard disk, without the introduction of any special hardware. Under NetWare this option is called "Spanning a volume" and is achieved when creating a new volume either at installation time or when adding another drive. Under Windows NT 3.5 this is a standard feature of the Disk Administrator and is called "Create Stripe Set".
The simplest approach to high reliability is to provide redundancy, which means that each volume has a secondary, duplicated copy. Each write is done to the primary and to the secondary volume. However, this simplest option is also the most expensive one, because it doubles costs. In addition, the throughput is lower because of higher control unit use. RAID-1 can employ "seek optimisation" – that is, the control unit can choose between two disks that contain the same data. By choosing the volume that results in the shorter latency delay, the average service time can drop RAID 1 is also known as mirroring or shadowing. In its simplest form two disks are connected to a single controller, and as data is written to one disk the same information is also written to the mirrored disk. If one drive fails, the system continues to run using data from the remaining drive. However, if the controller fails, both drives fail and the system is down. To overcome this, two controller cards can be used, which is a method known as duplexing. The biggest disadvantage of mirroring is its cost. With each active drive duplicated exactly, there is 50 percent data redundancy and twice the cost of the same system with no mirroring. Again, RAID 1 can be implemented on any NetWare or Windows NT system with more than 1 hard disk, without the introduction of any special hardware.
This is a mixture of both RAID-1 and RAID-0. It allows for data stripping and mirroring. The technique of error correction by hamming code, used in RAID-2, has long been used in semiconductor memory. Here, along with the 64 data bits, an additional 8 bits of ECC (Error Correction Code) codes are stored. It is impractical to implement the same algorithm on SCSI clusters. Consequently RAID-2 remains of academic interest only. With RAID-3, the data is stripped in parallel across several disks, and error correction information is stored on a dedicated disk called a "parity disk". The striping can be performed on a block, byte or bit level. The advantage of this model lies in its high potential data rate and low number (one) of parity devices. The main disadvantage is that the entire cluster is seen as a single device. Because RAID-3 is most suitable for applications that write or read large sequential data files and require high data transfer rates (for example, numerically intensive computing, modelling, visualisation and fluid dynamics). RAID-3 is not very useful for commercial transaction processing. RAID 3 employs a technique called 'disk striping with redundancy'. This offers the advantage of disk striping but without the very high level of data redundancy and cost of mirroring. It provides data redundancy by producing 'parity' data that is stored on a dedicated drive referred to as the parity drive. Parity data is created by applying complex mathematical calculations to the original data which extract relevant bits of information that can be stored using a fraction of the space used by a mirror image. In the event of a drive failure, this parity information can be used to reconstruct the original data. The disadvantage with this technique is that every time a write occurs, parity information must be stored to a single disk. The idea of introducing "Striping" is so that when a system is under heavy load, the writes are spread across a number of drives in parallel, implementing RAID 3 therefore reduces the performance of the system by having only a single drive for parity. The RAID-4 model improves performance of small transfers. A block of information is written to a single disk, and error correction is stored on a dedicated parity drive. Writing new data means writing new parity information but it is not mandatory to calculate the error correction information by reading the whole disk.
RAID-5 removes the bottleneck of RAID-4 by distributing the data and parity over the drives within the group. If a group contains three 1.0GB drives, the group data capacity will be 2.0GB, user data and error correction data will be interleaved and striped across all disks. To avoid all mix-up RAID-5 does not necessarily mean that 5 disks are needed, dependent of the vendor of the RAID system, a minimum of three disks are required. This system is considered most suitable for transaction processing in commercial applications RAID 5 combines data striping and the storing of parity data on all drives, as opposed to one dedicated parity drive. This option requires a minimum of three hard disk drives. RAID 5 and RAID 3 are very similar in terms of the benefits they offer, their cost implications and the way in which they function. However, there are some fundamental differences that make them suitable for very different applications. The process of storing parity data across a number of drives, rather than one dedicated parity drive, alleviates the bottleneck created by many parity write requests being directed to one drive. In a RAID 5 configuration the amount of data actually stored on each individual drive during the data striping process is larger than in RAID 3. Consequently requests are typically serviced from one drive, as opposed to several drives, and thus the potential competition for access to one drive is minimised. Due to this process RAID 5 is particularly effective for applications which access a high number of small files such as large databases and transaction processing systems (which most network users will be running). In contrast, RAID 3 is particularly effective for applications such as image processing which access large files. RAID 6 is very similar to RAID 5, except that it stores two copies of the parity data rather than one, with each copy being written to a separate drive. This feature significantly improves reliability as three disks in the array must fail for data to be lost; however, the write performance is the lowest of all RAIDs. It should be noted that some manufacturers use the term RAID 6 to define the combination of RAID 0 & 1. Level 10, usually pronounced "one zero", combines the features of RAID level 0 (striping) and RAID level 1 (mirroring) to produce a system with very high performance and high data reliability, but at a relatively high cost.
Q. What does RAID stand for ? A. RAID stands for Redundant Array of Inexpensive Disks, or more recently Redundant Array of Independent Disks Q. Why is RAID beneficial ? A. It can provide an increased degree of fault tolerance to prevent loss of information in the event of a failure of the storage sub-system. Q. Are there different types of RAID implementation ? A. There are 6 main types of RAID, known as levels 0 to 5, although in practice only levels 0, 1, 3, and 5 are used. Q. Is one particular type of RAID best ? A. The simple answer is 'no'. The most suitable implementation depends on the type of data that is likely to be processed. Q. Which RAID implementation is best for each type of data ? A. In general RAID 0 is used where speed is the only consideration as no redundancy is provided. RAID 1 is excellent where the need for reliability and speed are paramount; however, the cost of data storage cannot be a consideration since the amount of disk space is doubled. RAID 5 is best where there are small files with a lot of disk I/O such as large databases or transaction processing systems. Hot Swap is not a facility offered by RAID but is dependant upon the server itself. It requires the ability to power down each hard disk drive independently without shutting down the server. Once the drive is powered down then the it can be removed without putting too much electrical interference on the, still active, SCSI bus. This is a facility provided by the RAID controller card. If an additional disk is placed on the SCSI bus but is not used for any data, then in the event of one of the other drives failing, the data and parity information is recreated onto the "spare" drive thus allowing for more than one drive to fail without data loss. With RAID hardware and software installed, it is possible to continue normal operation in the event of a disk failure to rectify faults without downing the server. However, in any RAID scenario, whilst the system is running in "degraded" mode (ie. one of the disks has failed and it has not yet updated any hot spare drives that may be available) switching off the wrong drive can cause data loss. It is vital that all hardware engineers who may need to change drives in a RAID system have training on what effects removing the wrong drive can have. |
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