Redundant Arrays of independent disks is a family of techniques that use multiple disks that are organized to provide high performance and/or reliability

1. DARSHAN SINHGAD COLLEGE

2. • RAID (REDUNDANT ARRAY OF INDEPENDENT
DISKS) IS A FAMILY OF TECHNIQUES THAT USE
MULTIPLE DISKS (CALLED AN ARRAY OF DISKS)
THAT ARE ORGANIZED TO PROVIDE HIGH
PERFORMANCE AND/OR RELIABILITY
• The term "RAID" was first defined by David
Patterson, Garth A. Gibson, and Randy Katz at the
University of California, Berkeley in 1987.

3. Redundant array of independent disks,
originally Redundant array of inexpensive disks.
In the past system designers viewed storage systems
composed of several small cheap disk as a cost
effective alternative to using large disks.
Today however all disks are physically small and larger-
capacity disks actually have a lower cost per megabyte.
So now RAID systems are used for their higher
reliability and higher performance rate, rather than for
economic reason.

4. • The chance that some disk out of a set of N disks will fail is much
higher than the chance that a specific single disk will fail.
• Suppose that the Mean – time – to – failure (MTTF) of a single
disk is 100,000 hours . Then the MTTF of some disk in an array
of 100 disks will be 100,000/100 = 1,000 hours, or 41.66 days,
which is not long at all!
• If we store only one copy of data, then each disk failure will
result in loss of a significant amount of data – and such high rate
of data loss is unacceptable in mission critical systems.
• RAID system stores information that enables the system to
recover from errors , a technique called redundancy. A simplest
way to provide redundancy is DISK MIRRORING.

5. The solution to the problem of reliability to introduce
redundancy.
We store extra information that is not needed normally
but that can be used in the event of failure of a disk to
rebuild the lost information.
Data are not lost even if a disk fail.
The effective mean time to failure is increased.
Mirroring is a simplest way to introduced redundancy.

6. •Duplicate every disk.
•Logical disk consists of two physical disks.
•Every write is carried out on both disks.
•If one of the disk fails, data read from the other
•Data permanently lost only if the second disk fails before
the first failed disk is replaced.
•It increase mean time to data loss.
•Main Disadvantage :
--- Most expensive approach.

7. • Each RAID level is characterized by Data Striping.
•Data striping entails dividing storage into fixed size blocks called
strips.
• Contiguous strips of files are typically placed on separate disks
so that request for file data can be serviced using multiple disks
at once which improves access time.
•Each strip placed on same location on each disk in the array.
•Striping distributes a system’s data across multiple disks, which
enables high throughput than a single disk system because data
can be accessed from multiple disks simultaneously.
• It is called as Bit Level striping
•In Block Level striping blocks of files are striped across several
disks.

8. 1. RAID LEVEL – 0 (NON REDUNDANT STRIPING)
2. RAID LEVEL – 1 (MIRRORED DISKS)
3. RAID LEVEL – 2 (MEMORY STYLE ECC)
4. RAID LEVEL – 3 (BIT INTERLEAVED PARITY)
5. RAID LEVEL – 4 (BLOCK INTERLEAVED PARITY)
6. RAID LEVEL – 5 (BLOCK INTERLEAVED DISTRIBUTED PARITY)
7. RAID LEVEL – 6 ( P+Q REDUNDANCY SCHEME)
8. RAID LEVEL 0+1

9. It splits data among two or more disks.
It uses a striped disk array with no redundancy
RAID level 0 is simple to implement and does not
incur storage overhead to provide fault tolerance
RAID 0 system with n disks performs reads and
writes at a rate up to n times greater than that of
single disk.
RAID 0 systems are appropriate for systems where
high performance and low cost are more important
than reliability.

10. Disadvantages:
Reliability Problems : 1.No mirroring or parity bits
2.It does not provide fault
tolerance

11. • RAID level 1 employs disk mirroring (shadowing) to
provide redundancy.
•Each disk in an array is duplicated
•Stripes are not implemented at level 1
•Level 1 array permits multiple i/o operations to be serviced
simultaneously
•To ensure consistency modified data must be written to
mirrored pair
•Provides highest degree of fault tolerance
•Advantages : 1.system can sustain failures without loss of
data 2. Data regeneration (recovery & rebuilding is easy)
•3.some systems contains HOT SPARE DISKS that replace
failed disk, much like spare tire for an automobile.

12. DISADVANTAGES :
1.High storage overhead
2.Slower average write transfer rate
3.High cost
4. Twice as many disks are required to store the same data when
compared to RAID 0.

13. 1.RAID level 2 arrays are striped at the bit level so each
strip stores one bit
2.Level 2 arrays are not mirrored ,which reduces storage
overhead incur by level 1 arrays
3. Use parity bits to detect error and correct it.
4. Each byte in a memory system may have a parity bit
associated with it that records whether the numbers of
bits in the byte that are set to 1 is even or odd.
5.Also known as Memory – style error – correcting – code
(ECC)

14. Advantage:
--It requires less extra disks compare to level 1.
--Redundancy increase.

15. 1.RAID 3 stripes data at the bit or byte level
2.Instead of Hamming ECC RAID 3 uses XOR ECC’s which is
simpler than hamming ECC
3.It uses only one disk to hold parity information
regardless of size of the array.
4.Due to parity generation only one write can be
performed at a time, this yields high transfer rates when
reading or writing large files.
5.Primary advantage is that it is easy to implement , offers
reliability similar to level 2 RAID .

16. Advantages:
• Less Expensive.
• It needs only one parity bit.
Disadvantage:
• Expense of computing and writing parity
• Need to include a dedicated parity hardware.
• Poor I/O operations per sec.

17. • Stripes data at a block level across several
drives, with parity stored on one drive - block-
interleaved parity
• Allows recovery from the failure of any of the
disks
• Performance is very good for reads
• Writes require that parity data be updated each
time. Slows small random writes but large writes
are fairly fast

18. Advantage:
-- If 1 of the disks fails, It can be Restore using
parity block.
-- Multiple read accesses can be done on faster
rate.
Disadvantage:
-- A single write requires 4 disk accesses : 2 to read
old blocks and 2 to write blocks.

19. 19
• Spreads data and parity among all N+1 disks, rather than
storing data in N disks and parity in 1 disk
• Avoids potential overuse of a single parity disk –
improvement over RAID 4
• Most common parity RAID system

20. Advantages:
-- It increase the total number of requests that can be meet in a given amount
of time.
-- MTBF is slightly better than RAID 0. This is because failure of one disk is not
quite a harm. We need more time if 2 or more disks fail.
-- Performance is also as good as RAID 0, if not better. We can read and write
parallel blocks of data.
-- Pretty useful for general purpose uses where ‘read’s’ are more frequent the
write’s.
Disadvantage:
-- One of the drawbacks is that the write involves heavy parity calculations by
the RAID controller. Write operations are slower compared to RAID 0.

21. RAID LEVEL READ
CONCURRENCY
WRITE
CONCURRENCY
REDUNDANCY STRIPING
LEVEL
0 YES YES NONE BLOCK
1 YES NO MIRRORING NONE
2 NO NO HAMMING ECC
PARITY
BIT
3 NO NO XOR ECC PARITY BIT/BYTE
4 YES NO XOR ECC PARITY BLOCK
5 YES YES DISTRIBUTED
XOR ECC PARITY
BLOCK
COMPARISON OF RAID LEVEL 0 - 5

22. OTHER RAID LEVELS
RAID 6 P+Q :
Much like RAID level 5.
Store extra redundant information to guard against multiple
disk failure.
Uses error correcting codes, instead of parity.
RAID LEVEL 0+1 :-
A SET OF STRIPED DISKS (LEVEL 0) WHOSE IMAGE IS MIRRORED TO A
SECOND SET OF DISKS ( LEVEL 1).
RAID LEVEL 10 :-
A SET OF MIRRORED DATA (LEVEL 1) THAT IS STRIPED ACROSS ANOTHER
SET OF DISKS REQUIRING A MINIMUM OF 4 DISKS
OTHER RAID LEVELS DEVELOPED INCLUDING LEVELS 0+3, 0+5, 50, 1+5,51
AND 53

23. RAID 0 – High-Performance applications where data loss
is not critical
RAID 1 – High Reliability with fast recovery
RAID 2 and 4 – are subsumed by RAID levels 3 and 5.
RAID 3 – Bit striping is rarely used in any organization.
RAID 5 – Preferred for storing large volumes of data.
RAID 6 – Not Supported currently by many RAID
implementations.

24.  Another issue in the choice of RAID implementation is at the
level of hardware.
 Building special purpose hardware is significant benefit.
 System with special hardware support are called hardware
RAID systems.
 Using only software modification with no change at the
hardware level, such RAID implementations are called
software RAID.
 Hot swapping : - faulty disk can be removed and replaced
by new ones without turning power off, and reduce the mean
time to repair.

26. REFERENCES
1. OPERATING SYSTEMS –DEITEL,CHOFFNES
2. OPERATING SYSTEM PRINCIPLES –
SILBERSCHATZ,GALVIN,GAGNE
3. Database system concept 4th edition by, KORTH
4. Wikidedia
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