1. Topic: RAID
Levels – 0,1,2,3,4,5,6

2. RAID
Redundant Arrays Of
Independent Disks

3. Content:
 What is RAID?
 History behind RAID
 Taxonomy of RAID levels
 RAID : Level 0 to Level 6
 What that level means & does?
 Working with help of diagram
 Characteristics & Advantages
 Disadvantages
 Recommended Applications
 Benefits of using RAID Level
 References

4. What is RAID?
 It is multiple-disk database design.
 It is a category of disk drives (two or more) in combination for fault
tolerance and performance.
 It has seven levels – zero to six
 RAID disk drives are used frequently on servers but aren't generally
necessary for personal computers.
 RAID allows you to store the same data redundantly (in multiple
paces) in a balanced way to improve overall storage performance.

5. History of RAID:
 In 1987, Patterson, Gibson and Katz at the University of
California at Berkeley, published a paper entitled
“A Case for Redundant Arrays of Inexpensive Disks
(RAID)”
 I/O becoming a performance bottleneck

6. Different RAID Levels:
 Different levels which provides a different balance between
performance, capacity and tolerance.
 Following are they:

7. LEVEL 2
LEVEL 3
LEVEL 0
LEVEL 1
LEVEL 4
LEVEL 6
LEVEL 5
Striped Disk Array without Fault
Tolerance
Mirroring and Duplexing
Error-Correcting Coding
Bit-Interleaved Parity
Dedicated Parity Drive
Block Interleaved Distributed Parity
Independent Data Disks with Double
Parity

8. LEVEL 0: Striped Disk Array without Fault Tolerance-
 Minimum nos. of drives required : 2
 Striping is the segmentation of logically sequential data, such as
a single file, so that segments can be assigned to multiple physical
devices
 The first byte of the file is sent to the first drive, then the second
to second drive and so on.
 Stripes data across multiple disks without any redundant
information.
 Striping reduces the level of data availability since a disk failure
will cause the entire array to be inaccessible.

9. Diagram:
http://www.acnc.com/raidedu/0

10. Characteristics & 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 involved
 Very simple design
 Easy to implement

11. 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

12. Recommended Applications:
 Video Production and Editing
 Image Editing
 Pre-Press Applications
Any application requiring high bandwidth

13. LEVEL 1: Mirroring and Duplexing -
 Minimum nos. of drives required : 2
 RAID-1 provides data redundancy.
 Data written to one disk drive is simultaneously written to
another disk drive, called the mirroring.
 RAID-1 provides high data availability
 In addition, read performance may be enhanced if the array
controller allows simultaneous reads from both members of a
mirrored pair.
 Higher availability will be achieved if both disks in a
mirror pair are on separate I/O busses, known as
duplexing.

14. Diagram:
http://www.acnc.com/raidedu/1

15. Characteristics & 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

16. 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"

17. Recommended Applications:
 Accounting
 Payroll
 Financial
 Any application requiring very high availability

18. LEVEL 2: Error-Correcting Coding -
 It is a theoretical entity.
 It stripes data at bit level across an array of disks, then writes
check bytes to other disks in the array.
 The check bytes are calculated using a Hamming code.
 Theoretical performance is very high, but it would be so
expensive to implement that no-one uses it.

19. Characteristics & 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

20. Disadvantages:
 Very high ratio of ECC disks to data disks with smaller
word sizes – inefficient
Entry level cost vey 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

21. LEVEL 3: Bit-Interleaved Parity -
 Minimum nos. of drives required : 3
 A block of data is striped over an array of disks, then parity
data is written to a dedicated parity disk.
Successful implementations usually require that all the disks
have synchronized rotation.
RAID3 is very effective for large sequential data, such as
satellite imagery and video.

22. Diagram:
http://www.acnc.com/raidedu/3

23. Characteristics & 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

24. Disadvantages:
 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

25. Recommended Applications:
 Video Production and live streaming
 Image Editing
 Video Editing
 Prepress Applications
 Any application requiring high throughput

26. LEVEL 4: Dedicated Parity Drive -
 Minimum nos. of drives required : 3
 Level 4 provides block-level striping (like Level 0) with a parity
disk.
 If a data disk fails, the parity data is used to create a replacement
disk.

27. Diagram:
http://www.acnc.com/raidedu/4

28. Characteristics & Advantages:
 Very high Read data transaction rate
 Low ratio of ECC (Parity) disks to data disks means high efficiency
 High aggregate Read transfer rate

29. 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

30. Recommended Applications:
 Video Production and Editing
 Image Editing
 Pre-Press Applications
Any application requiring high bandwidth

31. LEVEL 5: Block Interleaved Distributed Parity -
 Minimum nos. of drives required : 3
 It distributes parity along with the data and requires that all drives
but one be present to operate. The array is not destroyed by a single
drive failure.
 On drive failure, any subsequent reads can be calculated from the
distributed parity such that the drive failure is masked from the end
user.

32. Example:
 Start with four drives
 Create an array using three of the physical drives, leaving the fourth as
a hot-spare drive.
 Then create a logical drive within that array.

33.  The data is striped across the drives, creating blocks.
 Notice that the storage of the data parity (denoted by *) also is striped,
and it shifts from drive to drive.
 A parity block ( *) contains a representation of the data from the other
blocks in the same stripe. For example, the parity block in the first
stripe contains data representation of blocks 1 and 2.

34.  If a physical drive fails in the array, the data from the failed physical
drive is reconstructed onto the hot-spare drive.

35. Characteristics & 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

36. 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

37. Recommended Applications:
 File and Application servers
 Database servers
 Web, E-mail, and News servers
 Intranet servers
 Most versatile RAID level

38. LEVEL 6: Independent Data Disks with Double Parity -
 Minimum nos. of drives required : 2
RAID 6 provides fault tolerance up to two failed drives.
This makes larger RAID groups more practical, especially for high-
availability systems. This becomes increasingly important as large-
capacity drives lengthen the time needed to recover from the failure of
a single drive.

39. Example:
 Start with six physical drives.
 Create a logical drive using four physical drives, leaving two for hot
spare drives.

40.  The data is striped across the drives, creating blocks in the logical drive.
The storage of the data parity (denoted by * and **) is striped, and it
shifts from drive to drive as it does in RAID level-5.
 If a physical drive fails in the array, the logical drive is degraded but
remains fault tolerant.

41.  If a second physical drive fails in the array, the data from the failed
drives are reconstructed onto the hot-spare drives, and the data for the
logical drive return to the original striping scheme.

42. Characteristics & 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 (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

43. Disadvantages:
 More complex controller design
 Controller 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

44. Recommended Applications:
 File and Application servers
 Database servers
 Web, E-mail, and News servers
 Intranet servers
 Excellent fault-tolerance with the lowest overhead

45. Table: RAID Levels

46. Benefits of RAID:
Data loss can be very dangerous for an organization
RAID technology prevents data loss due to disk failure
RAID technology can be implemented in hardware or software
Servers make use of RAID technology

47. Conclusion:
 RAID offers a cost effective alternative to SLED through the use of
 data striping
 mirroring
 parity
 Different RAID “levels” can be chosen to suit different functions
for the computer.

48. Reference:
 http://www.webopedia.com/TERM/R/RAID.html
 http://www.acnc.com/raid
 http://www.lascon.co.uk/hwd-raid.php