hraeissi
یک شنبه 23 آبان 1389, 10:12 صبح
[LEFT]This article is dedicated to learning almost all Types of RAIDs.
________________________RAID0_____________________ _________
RAID 0
Common Name(s): RAID 0. (Note that the term "RAID 0" is sometimes used to mean not only the conventional striping technique described here but also other "non-redundant" ways of setting up disk arrays. Sometimes it is (probably incorrectly) used just to describe a collection of disks that doesn't use redundancy.)
Technique(s) Used: Striping (without parity)
Description: The simplest RAID level, RAID 0 should really be called "AID", since it involves no redundancy. Files are broken into stripes of a size dictated by the user-defined stripe size of the array, and stripes are sent to each disk in the array. Giving up redundancy allows this RAID level the best overall performance characteristics of the single RAID levels, especially for its cost. For this reason, it is becoming increasingly popular by performance-seekers, especially in the lower end of the marketplace.
Controller Requirements: Supported by all hardware controllers, both SCSI and IDE/ATA, and also most software RAID solutions.
Hard Disk Requirements: Minimum of two hard disks (some may support one drive, the point of which escapes me); maximum set by controller. Any type may be used, but they should be of identical type and size for best performance and to eliminate "waste".
Array Capacity: (Size of Smallest Drive * Number of Drives).
Storage Efficiency: 100% if identical drives are used.
Fault Tolerance: None. Failure of any drive results in loss of all data, short of specialized data recovery.
Availability: Lowest of any RAID level. Lack of fault tolerance means no rapid recovery from failures. Failure of any drive results in array being lost and immediate downtime until array can be rebuilt and data restored from backup.
Degradation and Rebuilding: Not applicable.
Random Read Performance: Very good; better if using larger stripe sizes if the controller supports independent reads to different disks in the array.
Random Write Performance: Very good; again, best if using a larger stripe size and a controller supporting independent writes.
Sequential Read Performance: Very good to excellent.
Sequential Write Performance: Very good.
Cost: Lowest of all RAID levels.
Special Considerations: Using a RAID 0 array without backing up any changes made to its data at least daily is a loud statement that that data is not important to you.
Recommended Uses: Non-critical data (or data that changes infrequently and is backed up regularly) requiring high speed, particularly write speed, and low cost of implementation. Audio and video streaming and editing; web servers; graphic design; high-end gaming or hobbyist systems; temporary or "scratch" disks on larger machines.
_________________________________RAID1____________ ________________________
RAID Level 1
Common Name(s): RAID 1; RAID 1 with Duplexing.
Technique(s) Used: Mirroring or Duplexing
Description: RAID 1 is usually implemented as mirroring; a drive has its data duplicated on two different drives using either a hardware RAID controller or software (generally via the operating system). If either drive fails, the other continues to function as a single drive until the failed drive is replaced. Conceptually simple, RAID 1 is popular for those who require fault tolerance and don't need top-notch read performance. A variant of RAID 1 is duplexing, which duplicates the controller card as well as the drive, providing tolerance against failures of either a drive or a controller. It is much less commonly seen than straight mirroring.
Controller Requirements: Supported by all hardware controllers, both SCSI and IDE/ATA, and also most software RAID solutions.
Hard Disk Requirements: Exactly two hard disks. Any type may be used but they should ideally be identical.
Array Capacity: Size of Smaller Drive.
Storage Efficiency: 50% if drives of the same size are used, otherwise (Size of Smaller Drive / (Size of Smaller Drive + Size of Larger Drive) )
Fault Tolerance: Very good; duplexing even better.
Availability: Very good. Most RAID controllers, even low-end ones, will support hot sparing and automatic rebuilding of RAID 1 arrays.
Degradation and Rebuilding: Slight degradation of read performance; write performance will actually improve. Rebuilding is relatively fast.
Random Read Performance: Good. Better than a single drive but worse than many other RAID levels.
Random Write Performance: Good. Worse than a single drive, but better than many other RAID levels. )
Sequential Read Performance: Fair; about the same as a single drive.
Sequential Write Performance: Good; again, better than many other RAID levels.
Cost: Relatively high due to redundant drives; lowest storage efficiency of the single RAID levels. Duplexing is still more expensive due to redundant controllers. On the other hand, no expensive controller is required, and large consumer-grade drives are rather inexpensive these days, making RAID 1 a viable choice for an individual system.
Special Considerations: RAID 1 arrays are limited to the size of the drives used in the array. Multiple RAID 1 arrays can be set up if additional storage is required, but RAID 1+0 begins to look more attractive in that circumstance. Performance may be reduced if implemented using software instead of a hardware controller; duplexing may require software RAID and thus may show lower performance than mirroring.
Recommended Uses: Applications requiring high fault tolerance at a low cost, without heavy emphasis on large amounts of storage capacity or top performance. Especially useful in situations where the perception is that having a duplicated set of data is more secure than using parity. For this reason, RAID 1 is popular for accounting and other financial data. It is also commonly used for small database systems, enterprise servers, and for individual users requiring fault tolerance with a minimum of hassle and cost (since redundancy using parity generally requires more expensive hardware.)
_________________________________RAID2____________ _____________________
RAID Level 2
Common Name(s): RAID 2.
Technique(s) Used: Bit-level striping with Hamming code ECC.
Description: Level 2 is the "black sheep" of the RAID family, because it is the only RAID level that does not use one or more of the "standard" techniques of mirroring, striping and/or parity. RAID 2 uses something similar to striping with parity, but not the same as what is used by RAID levels 3 to 7. It is implemented by splitting data at the bit level and spreading it over a number of data disks and a number of redundancy disks. The redundant bits are calculated using Hamming codes, a form of error correcting code (ECC). Each time something is to be written to the array these codes are calculated and written oalng side the data to dedicated ECC disks; when the data is read back these ECC codes are read as well to confirm that no errors have occurred since the data was written. If a single-bit error occurs, it can be corrected "on the fly". If this sounds similar to the way that ECC is used within hard disks today, that's for a good reason: it's pretty much exactly the same. It's also the same concept used for ECC protection of system memory.
Level 2 is the only RAID level of the ones defined by the original Berkeley document that is not used today, for a variety of reasons. It is expensive and often requires many drives--see below for some surprisingly large numbers. The controller required was complex, specialized and expensive. The performance of RAID 2 is also rather substandard in transactional environments due to the bit-level striping. But most of all, level 2 was obviated by the use of ECC within a hard disk; essentially, much of what RAID 2 provides you now get for "free" within each hard disk, with other RAID levels providing protection above and beyond ECC.
Due to its cost and complexity, level 2 never really "caught on". Therefore, much of the information below is based upon theoretical analysis, not empirical evidence.
Controller Requirements: Specialized controller hardware required.
Hard Disk Requirements: Depends on exact implementation, but a typical setup required 10 data disks and 4 ECC disks for a total of 14, or 32 data disks and 7 ECC disks for a total of 39! The disks were spindle-synchronized to run in tandem.
Array Capacity: Depends on exact implementation but would be rather large if built today using modern drives.
Storage Efficiency: Depends on the number of data and ECC disks; for the 10+4 configuration, about 71%; for the 32+7 setup, about 82%.
Fault Tolerance: Only fair; for all the redundant drives included, you don't get much tolerance: only one drive can fail in this setup and be recoverable "on the fly".
Availability: Very good, due to "on the fly" error correction.
Degradation and Rebuilding: In theory, there would be little degradation due to failure of a single drive.
Random Read Performance: Fair. Bit-level striping makes multiple accesses impossible.
Random Write Performance: Poor, due to bit-level striping and ECC calculation overhead.
Sequential Read Performance: Very good, due to parallelism of many drives.
Sequential Write Performance: Fair to good.
Cost: Very expensive.
Special Considerations: Not used in modern systems.
Recommended Uses: Not used in modern systems.
__________________________________RAID3___________ ______________________
RAID Level 3
Common Name(s): RAID 3. (Watch out for some companies that say their products implement RAID 3 when they are really RAID 4.)
Technique(s) Used: Byte-level striping with dedicated parity.
Description: Under RAID 3, data is striped across multiple disks at a byte level; the exact number of bytes sent in each stripe varies but is typically under 1024. The parity information is sent to a dedicated parity disk, but the failure of any disk in the array can be tolerated (i.e., the dedicated parity disk doesn't represent a single point of failure in the array.) The dedicated parity disk does generally serve as a performance bottleneck, especially for random writes, because it must be accessed any time anything is sent to the array; this is contrasted to distributed-parity levels such as RAID 5 which improve write performance by using distributed parity (though they still suffer from large overheads on writes, as described here). RAID 3 differs from RAID 4 only in the size of the stripes sent to the various disks.
Controller Requirements: Generally requires a medium-to-high-end hardware RAID card.
Hard Disk Requirements: Minimum of three standard hard disks; maximum set by controller. Should be of identical size and type.
Array Capacity: (Size of Smallest Drive) * (Number of Drives - 1)
Storage Efficiency: If all drives are the same size, ( (Number of Drives - 1) / Number of Drives).
Fault Tolerance: Good. Can tolerate loss of one drive.
Availability: Very good. Hot sparing and automatic rebuild are usually supported by controllers that implement RAID 3.
Degradation and Rebuilding: Relatively little degrading of performance if a drive fails. Rebuilds can take many hours.
Random Read Performance: Good, but not great, due to byte-level striping.
Random Write Performance: Poor, due to byte-level striping, parity calculation overhead, and the bottleneck of the dedicated parity drive.
Sequential Read Performance: Very good.
Sequential Write Performance: Fair to good.
Cost: Moderate. A hardware controller is usually required, as well as at least three drives.
Special Considerations: Not as popular as many of the other commonly-implemented RAID levels. For transactional environments, RAID 5 is usually a better choice.
Recommended Uses: Applications working with large files that require high transfer performance with redundancy, especially serving or editing large files: multimedia, publishing and so on. RAID 3 is often used for the same sorts of applications that would typically see the use of RAID 0, where the lack of fault tolerance of RAID 0 makes it unacceptable.
___________________________________RAID4__________ ______________________
RAID Level 4
Common Name(s): RAID 4 (sometimes called RAID 3 by the confused).
Technique(s) Used: Block-level striping with dedicated parity.
Description: RAID 4 improves performance by striping data across many disks in blocks, and provides fault tolerance through a dedicated parity disk. This makes it in some ways the "middle sibling" in a family of close relatives, RAID levels 3, 4 and 5. It is like RAID 3 except that it uses blocks instead of bytes for striping, and like RAID 5 except that it uses dedicated parity instead of distributed parity. Going from byte to block striping improves random access performance compared to RAID 3, but the dedicated parity disk remains a bottleneck, especially for random write performance. Fault tolerance, format efficiency and many other attributes are the same as for RAID 3 and RAID 5.
_Controller Requirements: Generally requires a medium-to-high-end hardware RAID card.
Hard Disk Requirements: Minimum of three standard hard disks; maximum set by controller. Should be of identical size and type.
Array Capacity: (Size of Smallest Drive) * (Number of Drives - 1).
Storage Efficiency: If all drives are the same size, ( (Number of Drives - 1) / Number of Drives).
Fault Tolerance: Good. Can tolerate loss of one drive.
Availability: Very good. Hot sparing and automatic rebuild are usually supported..
Degradation and Rebuilding: Moderate degrading if a drive fails; potentially lengthy rebuilds.
Random Read Performance: Very good.
Random Write Performance: Poor to fair, due to parity calculation overhead and the bottleneck of the dedicated parity drive.
Sequential Read Performance: Good to very good.
Sequential Write Performance: Fair to good.
Cost: Moderate. A hardware controller is usually required, as well as at least three drives.
Special Considerations: Performance will depend to some extent upon the stripe size chosen.
Recommended Uses: Jack of all trades and master of none, RAID 4 is not as commonly used as RAID 3 and RAID 5, because it is in some ways a "compromise" between them that doesn't have a target market as well defined as either of those two levels. It is sometimes used by applications commonly seen using RAID 3 or RAID 5, running the gamut from databases and enterprise planning systems to serving large multimedia files.
__________________________________RAID5___________ _____________________
RAID Level 5
Common Name(s): RAID 5.
Technique(s) Used: Block-level striping with distributed parity.
Description: One of the most popular RAID levels, RAID 5 stripes both data and parity information across three or more drives. It is similar to RAID 4 except that it exchanges the dedicated parity drive for a distributed parity algorithm, writing data and parity blocks across all the drives in the array. This removes the "bottleneck" that the dedicated parity drive represents, improving write performance slightly and allowing somewhat better parallelism in a multiple-transaction environment, though the overhead necessary in dealing with the parity continues to bog down writes. Fault tolerance is maintained by ensuring that the parity information for any given block of data is placed on a drive separate from those used to store the data itself. The performance of a RAID 5 array can be "adjusted" by trying different stripe sizes until one is found that is well-matched to the application being used.
Controller Requirements: Requires a moderately high-end card for hardware RAID; supported by some operating systems for software RAID, but at a substantial performance penalty.
Hard Disk Requirements: Minimum of three standard hard disks; maximum set by controller. Should be of identical size and type.
Array Capacity: (Size of Smallest Drive) * (Number of Drives - 1).
Storage Efficiency: If all drives are the same size, ( (Number of Drives - 1) / Number of Drives).
Fault Tolerance: Good. Can tolerate loss of one drive.
Availability: Good to very good. Hot sparing and automatic rebuild are usually featured on hardware RAID controllers supporting RAID 5 (software RAID 5 will require down-time).
Degradation and Rebuilding: Due to distributed parity, degradation can be substantial after a failure and during rebuilding.
Random Read Performance: Very good to excellent; generally better for larger stripe sizes. Can be better than RAID 0 since the data is distributed over one additional drive, and the parity information is not required during normal reads.
Random Write Performance: Only fair, due to parity overhead; this is improved over RAID 3 and RAID 4 due to eliminating the dedicated parity drive, but the overhead is still substantial.
Sequential Read Performance: Good to very good; generally better for smaller stripe sizes.
Sequential Write Performance: Fair to good.
Cost: Moderate, but often less than that of RAID 3 or RAID 4 due to its greater popularity, and especially if software RAID is used.
Special Considerations: Due to the amount of parity calculating required, software RAID 5 can seriously slow down a system. Performance will depend to some extent upon the stripe size chosen.
Recommended Uses: RAID 5 is seen by many as the ideal combination of good performance, good fault tolerance and high capacity and storage efficiency. It is best suited for transaction processing and is often used for "general purpose" service, as well as for relational database applications, enterprise resource planning and other business systems. For write-intensive applications, RAID 1 or RAID 1+0 are probably better choices (albeit higher in terms of hardware cost), as the performance of RAID 5 will begin to substantially decrease in a write-heavy environment.
RAID6_____________________________________________ _______
RAID Level 6
Common Name(s): RAID 6. Some companies use the term "RAID 6" to refer to proprietary extensions of RAID 5; these are not discussed here.
Technique(s) Used: Block-level striping with dual distributed parity.
Description: RAID 6 can be thought of as "RAID 5, but more". It stripes blocks of data and parity across an array of drives like RAID 5, except that it calculates two sets of parity information for each parcel of data. The goal of this duplication is solely to improve fault tolerance; RAID 6 can handle the failure of any two drives in the array while other single RAID levels can handle at most one fault. Performance-wise, RAID 6 is generally slightly worse than RAID 5 in terms of writes due to the added overhead of more parity calculations, but may be slightly faster in random reads due to spreading of data over one more disk. As with RAID levels 4 and 5, performance can be adjusted by experimenting with different stripe sizes
Controller Requirements: Requires a specialized (usually meaning expensive) hardware controller.
Hard Disk Requirements: Minimum of four hard disks; maximum set by controller. Should be of identical size and type.
Array Capacity: (Size of Smallest Drive) * (Number of Drives - 2).
Storage Efficiency: If all drives are the same size, ( (Number of Drives - 2) / Number of Drives).
Fault Tolerance: Very good to excellent. Can tolerate the simultaneous loss of any two drives in the array.
Availability: Excellent.
Degradation and Rebuilding: Due to the complexity of dual distributed parity, degradation can be substantial after a failure and during rebuilding. Dual redundancy may allow rebuilding to be delayed to avoid performance hit.
Random Read Performance: Very good to excellent; generally better for larger stripe sizes.
Random Write Performance: Poor, due to dual parity overhead and complexity.
Sequential Read Performance: Good to very good; generally better for smaller stripe sizes.
Sequential Write Performance: Fair.
Cost: High.
Special Considerations: Requires special implementation; not widely available.
Recommended Uses: In theory, RAID 6 is ideally suited to the same sorts of applications as RAID 5, but in situations where additional fault tolerance is required. In practice, RAID 6 has never really caught on because few companies are willing to pay for the extra cost to insure against a relatively rare event--it's unusual for two drives to fail simultaneously (unless something happens that takes out the entire array, in which case RAID 6 won't help anyway). On the lower end of the RAID 5 market, the rise of hot swapping and automatic rebuild features for RAID 5 have made RAID 6 even less desirable, since with these advanced features a RAID 5 array can recover from a single drive failure in a matter of hours (where without them, RAID 5 would require downtime for rebuilding, giving RAID 6 a substantial advantage.) On the higher end of the RAID 5 market, RAID 6 usually loses out to multiple RAID solutions such as RAID 10 that provide some degree of multiple-drive fault tolerance while offering improved performance as well.
________________________________RAID7_____________ _______________________
RAID Level 7
Common Name(s): RAID 7.
Technique(s) Used: Asynchronous, cached striping with dedicated parity.
Description: Unlike the other RAID levels, RAID 7 isn't an open industry standard; it is really a trademarked marketing term of Storage Computer Corporation, used to describe their proprietary RAID design. (I debated giving it a page alongside the other RAID levels, but since it is used in the market, it deserves to be explained; that said, information about it appears to be limited.) RAID 7 is based on concepts used in RAID levels 3 and 4, but greatly enhanced to address some of the limitations of those levels. Of particular note is the inclusion of a great deal of cache arranged into multiple levels, and a specialized real-time processor for managing the array asynchronously. This hardware support--especially the cache--allow the array to handle many simultaneous operations, greatly improving performance of all sorts while maintaining fault tolerance. In particular, RAID 7 offers much improved random read and write performance over RAID 3 or RAID 4 because the dependence on the dedicated parity disk is greatly reduced through the added hardware. The increased performance of RAID 7 of course comes at a cost. This is an expensive solution, made and supported by only one company.
Controller Requirements: Requires a specialized, expensive, proprietary controller.
Hard Disk Requirements: Depends on implementation.
Array Capacity: Depends on implementation.
Storage Efficiency: Depends on implementation.
Fault Tolerance: Very good.
Availability: Excellent, due to use of multiple hot spares.
Degradation and Rebuilding: Better than many RAID levels due to hardware support for parity calculation operations and multiple cache levels.
Random Read Performance: Very good to excellent. The extra cache can often supply the results of the read without needing to access the array drives.
Random Write Performance: Very good; substantially better than other single RAID levels doing striping with parity.
Sequential Read Performance: Very good to excellent.
Sequential Write Performance: Very good.
Cost: Very high.
Special Considerations: RAID 7 is a proprietary product of a single company; if it is of interest then you should contact Storage Computer Corporation for more details on the specifics of implementing it. All the caching creates potential vulnerabilities in the event of power failure, making the use of one or more UPS units mandatory.
Recommended Uses: Specialized high-end applications requiring absolutely top performance and willing to live with the limitations of a proprietary, expensive solution. For most users, a multiple RAID level solution like RAID 1+0 will probably yield comparable performance improvements over single RAID levels, at lower cost.
________________________________RAID10(1+0 or 0+1)_________________________
RAID Levels 0+1 (01) and 1+0 (10)
Common Name(s): RAID 0+1, 01, 0/1, "mirrored stripes", "mirror of stripes"; RAID 1+0, 10, 1/0, "striped mirrors", "stripe of mirrors". Labels are often used incorrectly; verify the details of the implementation if the distinction between 0+1 and 1+0 is important to you.
Technique(s) Used: Mirroring and striping without parity.
Description: The most popular of the multiple RAID levels, RAID 01 and 10 combine the best features of striping and mirroring to yield large arrays with high performance in most uses and superior fault tolerance. RAID 01 is a mirrored configuration of two striped sets; RAID 10 is a stripe across a number of mirrored sets. RAID 10 and 01 have been increasing dramatically in popularity as hard disks become cheaper and the four-drive minimum is legitimately seen as much less of an obstacle. RAID 10 provides better fault tolerance and rebuild performance than RAID 01. Both array types provide very good to excellent overall performance by combining the speed of RAID 0 with the redundancy of RAID 1 without requiring parity calculations.
Controller Requirements: Almost all hardware controllers will support one or the other of RAID 10 or RAID 01, but often not both. Even low-end cards will support this multiple level, usually RAID 01. High-end cards may support both 01 and 10.
Hard Disk Requirements: An even number of hard disks with a minimum of four; maximum dependent on controller. All drives should be identical.
Array Capacity: (Size of Smallest Drive) * (Number of Drives ) / 2.
Storage Efficiency: If all drives are the same size, 50%.
Fault Tolerance: Very good for RAID 01; excellent for RAID 10.
Availability: Very good for RAID 01; excellent for RAID 10.
Degradation and Rebuilding: Relatively little for RAID 10; can be more substantial for RAID 01.
Random Read Performance: Very good to excellent.
Random Write Performance: Good to very good.
Sequential Read Performance: Very good to excellent.
Sequential Write Performance: Good to very good.
Cost: Relatively high due to large number of drives required and low storage efficiency (50%).
Special Considerations: Low storage efficiency limits potential array capacity.
Recommended Uses: Applications requiring both high performance and reliability and willing to sacrifice capacity to get them. This includes enterprise servers, moderate-sized database systems and the like at the high end, but also individuals using larger IDE/ATA hard disks on the low end. Often used in place of RAID 1 or RAID 5 by those requiring higher performance; may be used instead of RAID 1 for applications requiring more capacity.
hraeissi
یک شنبه 23 آبان 1389, 10:13 صبح
_______________________RAID0+3(03 or53)____________________
RAID Levels 0+3 (03 or 53) and 3+0 (30)
Common Name(s): The most confusing naming of any of the RAID levels. :^) In an ideal world, this level would be named RAID 0+3 (or 03) or RAID 3+0 (30). Instead, the number 53 is often used in place of 03 for reasons I have never been able to determine, and worse, 53 is often actually implemented as 30, not 03. As always, verify the details of the implementation to be sure of what you have.
Technique(s) Used: Byte striping with dedicated parity combined with block striping.
Description: RAID 03 and 30 (though often called 53 for a reason that utterly escapes me) combine byte striping, parity and block striping to create large arrays that are conceptually difficult to understand. :^) RAID 03 is formed by putting into a RAID 3 array a number of striped RAID 0 arrays; RAID 30 is more common and is formed by striping across a number of RAID 3 sub-arrays. The combination of parity, small-block striping and large-block striping makes analyzing the theoretical performance of this level difficult. In general, it provides performance better than RAID 3 due to the addition of RAID 0 striping, but closer to RAID 3 than RAID 0 in overall speed, especially on writes. RAID 30 provides better fault tolerance and rebuild performance than RAID 03, but both depend on the "width" of the RAID 3 dimension of the drive relative to the RAID 0 dimension: the more parity drives, the lower capacity and storage efficiency, but the greater the fault tolerance. See the examples below for more explanation of this.
Most of the characteristics of RAID 0+3 and 3+0 are similar to those of RAID 0+5 and 5+0. RAID 30 and 03 tend to be better for large files than RAID 50 and 05.
Controller Requirements: Generally requires a high-end hardware controller.
Hard Disk Requirements: Number of drives must be able to be factored into two integers, one of which must be 2 or higher and the other 3 or higher (you can make a RAID 30 array from 10 drives but not 11). Minimum number of drives is six, with the maximum set by the controller.
Array Capacity: For RAID 03: (Size of Smallest Drive) * (Number of Drives In Each RAID 0 Set) * (Number of RAID 0 Sets - 1). For RAID 30: (Size of Smallest Drive) * (Number of Drives In Each RAID 3 Set - 1) * (Number of RAID 3 Sets).
For example, the capacity of a RAID 03 array made of 15 18 GB drives arranged as three five-drive RAID 0 sets would be 18 GB * 5 * (3-1) = 180 GB. The capacity of a RAID 30 array made of 21 18 GB drives arranged as three seven-drive RAID 3 sets would be 18 GB * (7-1) * 3 = 324 GB. The same 21 drives arranged as seven three-drive RAID 3 sets would have a capacity of 18 GB * (3-1) * 7 = "only" 252 GB.
Storage Efficiency: For RAID 03: ( (Number of RAID 0 Sets - 1) / Number of RAID 0 Sets). For RAID 30: ( (Number of Drives In Each RAID 3 Set - 1) / Number of Drives In Each RAID 3 Set).
Taking the same examples as above, the 15-drive RAID 03 array would have a storage efficiency of (3-1)/3 = 67%. The first RAID 30 array, configured as three seven-drive RAID 3 sets, would have a storage efficiency of (7-1)/7 = 86%, while the other RAID 30 array would have a storage efficiency of, again, (3-1)/3 = 67%.
Fault Tolerance: Good to very good, depending on whether it is RAID 03 or 30, and the number of parity drives relative to the total number. RAID 30 will provide better fault tolerance than RAID 03.
Consider the two different 21-drive RAID 30 arrays mentioned above: the first one (three seven-drive RAID 3 sets) has higher capacity and storage efficiency, but can only tolerate three maximum potential drive failures; the one with lower capacity and storage efficiency (seven three-drive RAID 3 sets) can handle as many as seven , if they are in different RAID 3 sets. Of course few applications really require tolerance for seven independent drive failures! And of course, if those 21 drives were in a RAID 03 array instead, failure of a second drive after one had failed and taken down one of the RAID 0 sub-arrays would crash the entire array.
Availability: Very good to excellent.
Degradation and Rebuilding: Relatively little for RAID 30 (though more than RAID 10); can be more substantial for RAID 03.
Random Read Performance: Very good, assuming RAID 0 stripe size is reasonably large.
Random Write Performance: Fair.
Sequential Read Performance: Very good to excellent.
Sequential Write Performance: Good.
Cost: Relatively high due to requirements for a hardware controller and a large number of drives; storage efficiency is better than RAID 10 however and no worse than any other RAID levels that include redundancy.
Special Considerations: Complex and expensive to implement.
Recommended Uses: Not as widely used as many other RAID levels. Applications include data that requires the speed of RAID 0 with fault tolerance and high capacity, such as critical multimedia data and large database or file servers. Sometimes used instead of RAID 3 to increase capacity as well as performance.
vBulletin® v4.2.5, Copyright ©2000-1403, Jelsoft Enterprises Ltd.