RAID Is an Acronym for Redundant Array of Inexpensive (Or Independent) Disks
Various RAID Classes
RAID 0 (Striping)
RAID 1 (Mirroring)
RAID 0+1
RAID 2 (ECC)
RAID 3
RAID 4
RAID 5
RAID 6
RAID 7 (Proprietary)
RAID 10
RAID 1E
RAID 50 (same as RAID 05)
RAID 53
The "RAID" acronym first appeared in 1988 in the earliest of the Berkeley Papers written by Patterson, Gibson & Katz of the University of California at Berkeley. The RAID Advisory Board has since substituted "Independent" for "Inexpensive". A series of papers written by the original three authors and others defined and categorized several data protection and mapping models for disk arrays. Some of the models described in these papers, such as mirroring, were known at the time, others were new. The word levels used by the authors to differentiate the models from each other may suggest that a higher numbered RAID model is uniformly superior to a lower numbered one. This is not the case.
RAID 0 (Striping)
RAID 0: Striped Disk Array without Fault Tolerance
RAID Level 0 requires a minimum of 2 drives to implement.
RAID Level 0 is a performance oriented striped data mapping technique. Uniformly sized blocks of storage are assigned in regular sequence to all of an array's disks. RAID Level 0 provides high I/O performance at low inherent cost. (No additional disks are required). The reliability of RAID Level 0, however is less than that of its member disks due to its lack of redundancy. Despite the name, RAID Level 0 is not actually RAID, unless it is combined with other technologies to provide data and functional redundancy, regeneration and rebuilding.
Advantages: RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive. I/O performance is greatly improved by spreading the I/O load across many channels and drives. Best performance is achieved when data is striped across multiple controllers with only one drive per controller. No parity calculation overhead is involvedVery simple designEasy to implement.
Disadvantages: Not a "True" RAID because it is NOT fault-tolerant. The failure of just one drive will result in all data in an array being lost. Should never be used in mission critical environments. Recommended Applications? Video Production and Editing ? Image Editing ? Pre-Press Applications ? Any application requiring high bandwidth.
RAID 1 (Mirroring)
RAID 1: Mirroring and Duplexing. For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair.
RAID Level 1 requires a minimum of 2 drives to implement.
RAID Level 1, also called mirroring, has been used longer than any other form of RAID. It remains popular because of its simplicity and high level of reliability and availability. Mirrored arrays consist of two or more disks. Each disk in a mirrored array holds an identical image of user data. A RAID Level 1 array may use parallel access for high transfer rate when reading. More commonly, RAID Level 1 array members operate independently and improve performance for read-intensive applications, but at relatively high inherent cost. This is a good entry-level redundant system, since only two drives are required.
Advantages: One Write or two Reads possible per mirrored pair. Twice the Read transaction rate of single disks. Same write transaction rate as single disks. 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk. Transfer rate per block is equal to that of a single disk. Under certain circumstances, RAID 1 can sustain multiple simultaneous drive failures. Simplest RAID storage subsystem design.
Disadvantages: Highest disk overhead of all RAID types (100%) - inefficient. Typically the RAID function is done by system software, loading the CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended. May not support hot swap of failed disk when implemented in "software". Recommended Applications? Accounting ? Payroll ? Financial ? Any application requiring very high availability.
RAID 0+1
RAID 0+1: High Data Transfer Performance
RAID Level 0+1 requires a minimum of 4 drives to implement.
RAID Level 0+1 is a striping and mirroring combination without parity. RAID 0+1 has fast data access (like RAID 0), and single-drive fault tolerance (like RAID 1). RAID 0+1 still requires twice the number of disks (like RAID 1).
Advantages: RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays. RAID 0+1 has the same fault tolerance as RAID level 5. RAID 0+1 has the same overhead for fault-tolerance as mirroring alone. High I/O rates are achieved thanks to multiple stripe segments. Excellent solution for sites that need high performance but are not concerned with achieving maximum reliability.
Disadvantages: RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array. Very expensive / High overhead. All drives must move in parallel to proper track lowering sustained performance. Very limited scalability at a very high inherent cost. Recommended Applications? Imaging applications ? General fileserver.
RAID 2 (ECC)
RAID 2: Hamming Code ECC Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.
RAID Level 2 is one of two inherently parallel mapping and protection techniques defined in the Berkeley paper. It has not been widely deployed in industry largely because it requires special disk features. Since disk production volumes determine cost, it is more economical to use standard disks for RAID systems.
Advantages: "On the fly" data error correction. Extremely high data transfer rates possible. The higher the data transfer rate required, the better the ratio of data disks to ECC disks. Relatively simple controller design compared to RAID levels 3,4 & 5.
Disadvantages: Very high ratio of ECC disks to data disks with smaller word sizes - inefficient. Entry level cost very high - requires very high transfer rate requirement to justify. Transaction rate is equal to that of a single disk at best (with spindle synchronization). No commercial implementations exist / not commercially viable.
RAID 3
RAID 3: Parallel transfer with Parity The data block is subdivided ("striped") and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 3 requires a minimum of 3 drives to implement.
RAID Level 3 adds redundant information in the form of parity to a parallel access striped array, permitting regeneration and rebuilding in the event of a disk failure. One stripe of parity protects corresponding strip's of data on the remaining disks. RAID Level 3 provides for high transfer rate and high availability, at an inherently lower cost than mirroring. Its transaction performance is poor, however, because all RAID Level 3 array member disks operate in lockstep.
RAID 3 utilizes a striped set of three or more disks with the parity of the strips (or chunks) comprising each stripe written to a disk. Note that parity is not required to be written to the same disk. Furthermore, RAID 3 requires data to be distributed across all disks in the array in bit or byte-sized chunks. Assuming that a RAID 3 array has N drives, this ensures that when data is read, the sum of the data-bandwidth of N - 1 drives is realized. The figure below illustrates an example of a RAID 3 array comprised of three disks. Disks A, B and C comprise the striped set with the strips on disk C dedicated to storing the parity for the strips of the corresponding stripe. For instance, the strip on disk C marked as P(1A,1B) contains the parity for the strips 1A and 1B. Similarly the strip on disk C marked as P(2A,2B) contains the parity for the strips 2A and 2B.
Advantages: Very high Read data transfer rate. Very high Write data transfer rate. Disk failure has an insignificant impact on throughput. Low ratio of ECC (Parity) disks to data disks means high efficiency. RAID 3 ensures that if one of the disks in the striped set (other than the parity disk) fails, its contents can be recalculated using the information on the parity disk and the remaining functioning disks. If the parity disk itself fails, then the RAID array is not affected in terms of I/O throughput but it no longer has protection from additional disk failures. Also, a RAID 3 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.
Disadvantages: Transaction rate equal to that of a single disk drive at best (if spindles are synchronized). Read operations can be time-consuming when the array is operating in degraded mode. Due to the restriction of having to write to all disks, the amount of actual disk space consumed is always a multiple of the disks' block size times the number of disks in the array. This can lead to wastage of space. Controller design is fairly complex. Very difficult and resource intensive to do as a "software" RAID. Recommended Applications? Video Production and live streaming ? Image Editing ? Video Editing ? Prepress Applications ? Any application requiring high throughput.
RAID 4
RAID 4: Independent Data disks with Shared Parity disk Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 4 requires a minimum of 3 drives to implement.
Like RAID Level 3, RAID Level 4 uses parity concentrated on a single disk to protect data. Unlike RAID Level 3, however, a RAID Level 4 array's member disks are independently accessible. Its performance is therefore more suited to transaction I/O than large file transfers. RAID Level 4 is seldom implemented without accompanying technology, such as write-back cache, because the dedicated parity disk represents an inherent write bottleneck.
Advantages: Very high Read data transaction rate. Low ratio of ECC (Parity) disks to data disks means high efficiency. High aggregate Read transfer rate.
Disadvantages: Quite complex controller design. Worst Write transaction rate and Write aggregate transfer rate. Difficult and inefficient data rebuild in the event of disk failure. Block Read transfer rate equal to that of a single disk.
RAID 5
RAID 5: Independent Data disks with Distributed Parity blocks Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. The array capacity is N-1.
RAID Level 5 requires a minimum of 3 drives to implement.
By distributing parity across some or all of an array's member disks, RAID Level 5 reduces (but does not eliminate) the write bottleneck inherent in RAID Level 4. As with RAID Level 4, the result is asymmetrical performance, with reads substantially outperforming writes. To reduce or eliminate this intrinsic asymmetry, RAID level 5 is often augmented with techniques such as caching and parallel multiprocessors.
The figure below illustrates an example of a RAID 5 array comprised of three disks - disks A, B and C. For instance, the strip on disk C marked as P(1A,1B) contains the parity for the strips 1A and 1B. Similarly the strip on disk A marked as P(2B,2C) contains the parity for the strips 2B and 2C. RAID 5 ensures that if one of the disks in the striped set fails, its contents can be extracted using the information on the remaining functioning disks. It has a distinct advantage over RAID 4 when writing since (unlike RAID 4 where the parity data is written to a single drive) the parity data is distributed across all drives. Also, a RAID 5 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.
Advantages: Highest Read data transaction rate. Medium Write data transaction rate. Low ratio of ECC (Parity) disks to data disks means high efficiency. Good aggregate transfer rate.
Disadvantages: Disk failure has a medium impact on throughput. Most complex controller design. Difficult to rebuild in the event of a disk failure (as compared to RAID level 1). Individual block data transfer rate same as single disk. Recommended Applications? File and Application servers ? Database servers ? WWW, E-mail, and News servers ? Intranet servers ? Most versatile RAID level.
RAID 6
RAID 6: Independent Data disks with two Independent Distributed Parity schemes.
Advantages: RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (two-dimensional parity). Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives. RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures. Perfect solution for mission critical applications.
Disadvantages: Very complex controller design. Controller overhead to compute parity addresses is extremely high. Very poor write performance. Requires N+2 drives to implement because of two-dimensional parity scheme.