2. Topic
RAID
Redundant Array
Independent Disk

3. Raid History

4. S.L.E.D.
Single Large Expensive Disks
Single drive used to store data.
 Capacity: good
Problem:
Data cant be read and write quickly
 If SLED fail then all data loss.


5. Raid

6. RAID Overview:
The heart of the RAID storage system is
controller card. The task of the controller
card is to
Manage Individual Hard Disk Drives
Provide a Logical Array Configuration
Perform Redundant or Fault Tolerant
Operations

8. Raid

9. Patented 1987
Built in 1989
Updated several times
That’s all I could find. Until I made
these updates….

10. RAID
RAID: Redundant Arrays of
Independent Disks
Hence, the I in RAID now
stands for“independent”instead
of “inexpensive”.
RAID:Multiple disk drives
provides reliability via
redundancy.
commonly used to address
the performance and reliability
issues.

11. Redundancy:
Mirroring
Duplicate every disk
Gives good error recovery
Data stripping
A method of concatenating multiple
drives into one logical storage unit.
the data is split into different parts.
Parity:
Splitting data onto blocks with the
help of XOR operation

12. What exactly is a RAID?
RAID is basically drives stacked
on top of each other like a cake
with layers that can share their
data together.

13. Features Of RAID Levels:
RAID 0 – Data Striping
RAID 1 - Mirroring
RAID 2 – Hamming Code
RAID 3 – Single Check Disk per
Group
There are more lavels like 4,5,6 10.

14. RAID Level 0:

15. RAID level 0:
Simplest RAID implementation
Includes striping but no redundancy
Highest-performance
High risk of data loss
Multiple drives involved
Could lose all data in array with
one drive failure

16. Cont………..!!
Block level striping
Notes:
If two different I/O requests are
pending for two different blocks of
data, in all likelihood the two blocks are
located in two different disks, then the
requests can be issued in parallel
If a single request is spread across
multiple logically contiguous strips, the
request can be handled in parallel.

17. Recommended Applications
Video Production and Editing
Image Editing
Pre-Press Applications
gaming systems.
Disadvantage:
Relaibility problem –no mirroring or parity bits.
Advantage:
speed enhancement
Maximum utilization of physical drive storage
capacity, because no room is taken for redundant data
or data-parity storage

19. RAID Level1:

20. RAID Level 1:
Highest level of redundancy
Each drive has a mirrored copy in array
No striping at this level
Improves read performance over single
disks because multiple disks can be read at
once
Slower write performance because two
disks must be accessed for each modified
data item to maintain mirroring
High storage overhead
Only half array stores unique data
Most suitable where reliability is primary
concern.

21. Cont……..!!!!!!
Performance:
If we use independent
disk controllers for each
disk, then we can increase
the read or write speeds
by doing operations in
parallel.

22. Application:
Accounting
Financial
Advantage:
Provide best Performance
Provide Fault tolerance
Disadvantage:
High cost.
Requires twice the disk space

24. RAID Level 2:

25. RAID Level 2:
Implements redundancy via striping
Striped at bit level
Uses Hamming ECC to check data
integrity
ECC data stored on separate drive
Significant overhead in storage and
performance .
RAID 2 is the only RAID level
that can repair errors, the other
RAID levels can only detect them

26. Cont……….!!!!!
Read – all disks are
simultaneously accessed
Write - all disks are
simultaneously accessed
Write penalty – computation of
the Hamming ECC
Used when many disk errors
occur, but given the high reliability
of individual disks, rarely used.

27. Advantages:
Random Read Performance= Fair
Sequential Read Performance= Very Good
Sequential Write Performance= Very Good
Disadvantages
:
Random Write Performance= Poor
Requires a complex controller
High overhead for check disks
Not used in modern systems

29. RAID Level 3:

30. RAID Level 3:
Also stripes at the byte level
Uses XOR to calculate parity for ECC
Much simpler than Hamming ECC
Requires only one disk for parity
information regardless of the size of the
array
Cannot determine which bit contains error,
but this information can be gathered easily
by inspecting the array for a failed disk
High transfer rates, but only one request
serviced at a time

31. Cont………!!!
In the case of a disk failure,
All data are available 
missing data can be calculated
from the parity bit
Write: just maintain the parity
such that later it can be
regenerated.
Failed disk to be replaced and
the data regenerated

32. Cont….!!!!!!!!!!
BYTE level striping and XOR ECC allows
for one check disk: lowest overhead possible
Example…
A:0101 XOR B:0011 = Check:0110
A is gone?
B:0011 XOR Check:0110 = 0101 (A)