RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. A RAID array is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other. |
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 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: 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: 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: 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: 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: 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: 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.
RAID 7: Optimized Asynchrony for High I/O Rates as well as High Data Transfer Rates.
Architectural Features:· All I/O transfers are asynchronous, independently controlled and cached including host interface transfers· All Reads and Write are centrally cached via the high speed x-bus· Dedicated parity drive can be on any channel· Fully implemented process oriented real time operating system resident on embedded array control microprocessor· Embedded real time operating system controlled communications channel· Open system uses standard SCSI drives, standard PC buses, motherboards and memory SIMMs· High speed internal cache data transfer bus (X-bus)· Parity generation integrated into cache· Multiple attached drive devices can be declared hot standbys· Manageability: SNMP agent allows for remote monitoring and management.
Advantages: Overall write performance is 25% to 90% better than single spindle performance and 1.5 to 6 times better than other array levelsHost interfaces are scalable for connectivity or increased host transfer bandwidth. Small reads in multi user environment have very high cache hit rate resulting in near zero access times. Write performance improves with an increase in the number of drives in the array. Access times decrease with each increase in the number of actuators in the array. No extra data transfers required for parity manipulation. RAID 7 is a registered trademark of Storage Computer Corporation.
Disadvantages: One vendor proprietary solution. Extremely high cost per MB. Very short warranty. Not user serviceable. Power supply must be UPS to prevent loss of cache data.
RAID 10: Very High Reliability combined with High Performance
RAID Level 10 requires a minimum of 4 drives to implement.Advantages: RAID 10 is implemented as a striped array whose segments are RAID 1 arrays. RAID 10 has the same fault tolerance as RAID level 1. RAID 10 has the same overhead for fault-tolerance as mirroring alone. High I/O rates are achieved by striping RAID 1 segments. Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures. Excellent solution for sites that would have otherwise gone with RAID 1 but need some additional performance boost.
Disadvantages: 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· Database server requiring high performance and fault tolerance.
RAID 10 arrays are typically used in environments that require uncompromising availability coupled with exceptionally high throughput for the delivery of data located in secondary storage. In recent years a number of mutations of RAID 10 have been developed with similar capabilities. This paper presents one of the popular alternative implementations and discusses the relative advantages and disadvantages of RAID 10 and this alternative.
A RAID 10 array is formed using a two-layer hierarchy of RAID types. At the lowest level of the hierarchy are a set of RAID 1 sub-arrays i.e., mirrored sets. These RAID 1 sub-arrays in turn are then striped to form a RAID 0 array at the upper level of the hierarchy. The collective result is a RAID 10 array. The figure below demonstrates a RAID 10 comprised of two RAID 1 sub-arrays at the lower level of the hierarchy. They are sub-arrays A (comprised of disks A1 and A2) and B (comprised of disks B1 and B2). These two sub-arrays in turn are striped using the strips 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B to form a RAID 0 at the upper level of the hierarchy. The result is a RAID 10. Figure 1 illustrates a RAID 10 array, with each disk in the array participating in exactly one mirrored set, thereby forcing the number of disks in the array to be even.
Let us now look at some of the salient properties of RAID 10. Consider a RAID 10 comprised of d disks and N mirrored sets (i.e., constituent RAID 1 sub-arrays). Since each disk in the array participates in exactly one mirrored set, d = 2N.
(a) RAID 10 arrays do not require any parity calculation at any stage of their construction or operation.
(b) RAID 10 arrays are generally deployed in environments that require a high degree of redundancy. The ability to survive multiple failures is a fundamental property of RAID 10. In fact the maximum number of disk failures a RAID 10 array can withstand is d/2 = N.
What about the number of combinations of failed disks that a RAID 10 array can sustain? The number of ways in which k disks can fail is given by NCk ·2k, since there are NCk ways in which to choose k mirror groups from N possible choices, and 2 ways in which to choose a disk within each mirror group. Therefore the total number of combinations of failed disks that a RAID 10 can support is:
NC1 ·21 + NC2 ·22 + … + NCN ·2N
= (2 + 1)N - 1
= 3N - 1Thus, for a 4 drive RAID 10 containing 2 mirrored sets, the number of combinations in which disks can fail without the array being rendered inoperable is 32 - 1 = 8. In fact, these combinations may be enumerated as follows, with each possible set of failed disks listed within braces. They are: {A1}, {A2}, {B1}, {B2}, {A1, B1}, {A2, B2}, {A1, B2}, and {A2, B1}.
(c) RAID 10 ensures that if a disk in any constituent mirrored set fails, its contents can be extracted from the functioning disk in its mirrored set. Thus, when a RAID 10 array has suffered the maximum number of disk failures it is capable of withstanding, its throughput rate is no worse than that of a RAID 0 with N disks. In fact, any combination of N contiguous independent strips can be read concurrently. The term "independent strip" is used to denote a strip in a collection of strips that is not a mirror of any other strip within that collection.
(d) A RAID 10 array that is in a nominal state can improve the throughput of read operations by allowing concurrent reads to be performed on multiple disks in the array. For example, if the strips 1A, 1B, 2A, 2B are to be read from the array given in figure 1, it is clear that all four strips can be read concurrently from the disks A1, B1, A2 and B2 respectively.
RAID 1E: While RAID 10 has been traditionally implemented using an even number of disks, some hybrids can use an odd number of disks as well. Figure 2 illustrates an example of a hybrid RAID 10 array comprised of five disks; A, B, C, D and E. In this configuration, each strip is mirrored on an adjacent disk with wrap-around. In fact this scheme - or a slightly modified version of it - is often referred to as RAID 1E and was originally proposed by IBM. Let us now investigate the properties of this scheme.
When the number of disks comprising a RAID 1E is even, the striping pattern is identical to that of a traditional RAID 10, with each disk being mirrored by exactly one other unique disk. Therefore, all the characteristics for a traditional RAID 10 apply to a RAID 1E when the latter has an even number of disks. However, RAID 1E has some interesting properties when the number of disks is odd.
(a) Just as in the case of traditional RAID 10, RAID 1E does not require any parity calculation either. So in this category, RAID 10 and RAID 1E are equivalent.
(b) The maximum number of disk failures a RAID 1E array using d disks can withstand is d/2 . When d is odd, this yields a value that is the equal to that of a traditional RAID 10 while utilizing one additional disk. What about the number of combinations of disk failures that RAID 1E can support? It turns out that RAID 1E is very peculiar in this characteristic when d is odd. Assume for the sake of notational convenience that d/2 = p. Then the number of ways in which k disks can fail is d·P-1Ck-1, since there are d ways to choose the first disk and P-1Ck-1 ways to choose the remaining k-1 disks from p-1 possible choices. Therefore, the total number of combinations of failed disks that this scheme can support is:
d·p-1C0 + d·p-1C1 + ... + d·p-1Cp-1
= d · (p-1C0 + p-1C1 + … + p-1Cp-1)
= d · 2p-1Thus, for a 5 drive RAID 1E, the total number of combinations in which disks can fail without the array being rendered inoperable is 5·22-1 = 10. However, this result also indicates that as the value of d increases, the ratio of the number of combinations of disk failures supported by RAID 1E using d disks decreases with respect to conventional RAID 10 using d-1 disks. In fact, for d > 9, RAID 1E yields a lesser number of combinations! For instance, while a conventional RAID 10 using 10 disks can support 35 - 1 = 242 combinations of disk failures, RAID 1E using 11 disks can support only 11·25-1 = 176 combinations. Clearly, RAID 10 is a superior choice when tolerance to a larger number of combinations of disk failures is considered important. An even more significant implication of this result is the following. Since a RAID 1E with an even number of disks is identical to a traditional RAID 10, A RAID 1E with 10 disks can support more combinations of failures than a RAID 1E with 11 disks. In general, a RAID 1E with 2N disks can support more combinations of failures that a RAID 1E with 2N + 1 disks, when N 5. In other words, it is always preferable to utilize an even number of disks for your RAID 1E than an odd number if you desire a higher tolerance to disk failures. In other words, it is always preferable to use a traditional RAID 10!
(c) When a RAID 1E array suffers the maximum number of disk failures it is capable of withstanding, i.e., d/2 , the number of contiguous independent strips that can be accessed concurrently can be less than d/2 . For example, consider the RAID 1E array displayed in figure 2. Assume that disks A and C have failed. In this scenario, it is clear that the contiguous strips 4, 5 and 6 cannot be read concurrently although three disks remain operational. Thus the throughput of a RAID 1E with d disks - where d is odd - may be no higher under specific access patterns than that of a RAID 10 with d-1 disks when both arrays experience the maximum number of sustainable disk failures.
(d) Just as in the case of a traditional RAID 10 implementation, RAID 1E in a nominal state can improve the throughput of read operations by allowing concurrent reads to be performed on multiple disks in the array. The fact that there are more disks than there are mirror sets should intuitively suggest as much.
Conclusion: RAID 1E offers a little more flexibility in choosing the number of disks that can be used to constitute an array. The number can be even or odd. However, RAID 10 is far more robust in terms of the number of combinations of disk failures it can sustain even when using lesser number of disks. Furthermore, a RAID 10 guarantees a throughput rate that is always equal to that which is obtainable from the concurrent use of all its functioning disks. In contrast, specific access patterns may not lend themselves to the concurrent use of all functioning disks under RAID 1E. Therefore, if extremely high availability and throughput are of paramount importance to your applications, RAID 10 should be the configuration of choice.
RAID 50 array is formed using a two-layer hierarchy of RAID types. At the lowest level of the hierarchy is a set of RAID 5 arrays. These RAID 5 arrays in turn are then striped to form a RAID 0 array at the upper level of the hierarchy. The collective result is a RAID 50 array. The figure below demonstrates a RAID 50 comprised of two RAID 5 arrays at the lower level of the hierarchy – arrays X and Y. These two arrays in turn are striped using 4 stripes (comprised of the strips 1X, 1Y, 2X, 2Y, etc.) to form a RAID 0 at the upper level of the hierarchy. The result is a RAID 50.
Advantage: RAID 50 ensures that if one of the disks in any parity group fails, its contents can be extracted using the information on the remaining functioning disks in its parity group. Thus it offers better data redundancy than the simple RAID types, i.e., RAID 1, 3, and 5. Also, a RAID 50 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.
RAID 53: High I/O Rates and Data Transfer Performance
RAID Level 53 requires a minimum of 5 drives to implement.Advantages: RAID 53 should really be called "RAID 03" because it is implemented as a striped (RAID level 0) array whose segments are RAID 3 arrays. RAID 53 has the same fault tolerance as RAID 3 as well as the same fault tolerance overhead. High data transfer rates are achieved thanks to its RAID 3 array segments. High I/O rates for small requests are achieved thanks to its RAID 0 striping. Maybe a good solution for sites who would have otherwise gone with RAID 3 but need some additional performance boost.
Disadvantages: Very expensive to implement. All disk spindles must be synchronized, which limits the choice of drives. Byte striping results in poor utilization of formatted capacity
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