RAID

1. Redundant Array Of Inexpensive (Independent) Disks RAID

2. Introduction • A variety of disk organization techniques, collectively called RAID. • The basic idea behind a RAID is to install a box full of disks next to the computer which replace the disk controller card with a RAID controller. • It copies the data over to the RAID, and then continue normal operation. • Several different schemes for allowing parallel operations this were classified into levels are called RAID levels.

3. Key goals • Redundancy i.e. increased data reliability • Increased input/output performance • High data transfer rate • Used to address the performance • Error correction

4. Important Terms • Data Mirroring : Mirrored arrays write data to paired drives simultaneously. If one drive fails, the data is preserved on the paired drive. Mirroring provides data protection through redundancy. • Data Striping: Striping across disks allows data to be written and accessed on more than one drive, at the same time. Striping combines each drive's capacity into one large volume. Striped disk arrays achieve highest transfer rates and performance at the expense of fault tolerance.

5. Example of Data Striping • Bit Level Striping : Data Striping consists of splitting the bits of each byte across multiple disks; it’s called bit-level striping. • For example, if we have an array of eight disks, we write bit i of each byte to disk i. The array of eight disks can be treated as a single disk with sectors that are eight times the normal size, and, more important, that have eight times the access rate.

6. Example of Data Striping(cont.) • Block Level Striping : Blocks of a file are striped across multiple disks; with n disks, block i of a file goes to disk (i mod n) + 1. Other levels of striping, such as bytes of a sector or sectors of a block, are also possible.

7. RAID Levels • RAID 0 • RAID 1 • RAID 2 • RAID 3 • RAID 4 • RAID 5 • RAID 6 • RAID 10 • RAID 20 • RAID 50

8. RAID Level 0 • It is also known as disk stripping, because it uses a disk file system called a stripe set. • Data is divided into blocks and is spread in a fixed order among all the disks in the array. • RAID level 0 improves read and write performance by spreading operations across multiple disks, so that operations can be performed independently. • It doesn’t provide fault tolerance.

9. RAID 0 Architecture

10. RAID 0 (cont…) • RAID 0 implements a striped disk array i.e. the data is broken down into blocks and each block is written to a separate disk driveI/O performance is greatly improved by spreading the I/O load across many channels and drives • Very simple design • Easy to implement

11. Disadvantages of RAID 0 • 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

12. Applications of RAID 0 • Video Production and Editing • Image Editing • Pre-Press Applications • Any application requiring high bandwidth

13. RAID 1 • 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

14. RAID 1 Architecture

15. Disadvantages of RAID 1 • Highest disk overhead of all RAID types (100%) - inefficientTypically 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"

16. Applications of RAID 1 • Accounting • Payroll • Financial • Any application requiring very high availability

17. RAID 2 • "On the fly" data error correctionExtremely 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

18. Disadvantages of RAID 2 • Very high ratio of ECC disks to data disks with smaller word sizes - inefficientEntry 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

19. RAID 3 • 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

20. RAID 3 Architecture

21. RAID 3 (cont…) • Very high Read data transfer rateVery 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

22. Disadvantages of RAID 3 • Transaction rate equal to that of a single disk drive at best (if spindles are synchronized)Controller design is fairly complex • Very difficult and resource intensive to do as a "software" RAID

23. Applications of RAID 3 • Video Production and live streaming • Image Editing • Video Editing • Prepress Applications • Any application requiring high throughput

24. RAID 4 • 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

25. RAID 4 (cont…) • Very high Read data transaction rateLow ratio of ECC (Parity) disks to data disks means high efficiency • High aggregate Read transfer rate

26. RAID 4 Architecture

27. Disadvantages of RAID 4 • Quite complex controller designWorst 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

28. RAID 5 • 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. • RAID Level 5 requires a minimum of 3 drives to implement

29. RAID 5(cont…) • Highest Read data transaction rateMedium Write data transaction rate • Low ratio of ECC (Parity) disks to data disks means high efficiency • Good aggregate transfer rate

30. Disadvantages of RAID 5 • Disk failure has a medium impact on throughputMost 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

31. Applications • File and Application servers • Database servers • Web, E-mail, and News servers • Intranet servers • Most versatile RAID level

32. RAID 6 • Two independent parity computations must be used in order to provide protection against double disk failure. Two different algorithms are employed to achieve this purpose. • RAID Level 6 requires a minimum of 4 drives to implement

33. RAID 6 (cont…) • RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (dual 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 • RAID 6 protects against multiple bad block failures while non-degraded • RAID 6 protects against a single bad block failure while operating in a degraded mode • Perfect solution for mission critical applications

34. Disadvantages of RAID 6 • More complex controller designController overhead to compute parity addresses is extremely high • Write performance can be brought on par with RAID Level 5 by using a custom ASIC for computing Reed-Solomon parity • Requires N+2 drives to implement because of dual parity scheme

35. Applications of RAID 6 • File and Application serversDatabase serversWeb and E-mail serversIntranet serversExcellent fault-tolerance with the lowest overhead

36. RAID 10 • RAID Level 10 requires a minimum of 4 drives to implement • RAID 10 is implemented as a striped array whose segments are RAID 1 arraysRAID 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

37. Disadvantages of RAID 10 • 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

38. Applications of RAID 10 • Database server requiring high performance    and fault tolerance

39. RAID 50 • RAID Level 50 requires a minimum of 6 drives to implement • RAID 50 should have been called "RAID 03" because it was implemented as a striped (RAID level 0) array whose segments were RAID 3 arrays (during mid-90s)Most current RAID 50 implementation is illustrated above • RAID 50 is more fault tolerant than RAID 5 but has twice the parity overhead

40. RAID 50 (cont…) • High data transfer rates are achieved thanks to its RAID 5 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 5 but need some additional performance boost

41. Disadvantages of RAID 50 • Very expensive to implementAll disk spindles must be synchronized, which limits the choice of drives • Failure of two drives in one of the RAID 5 segments renders the whole array unusable

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