disk structure and Raid levels

1. Disks are potential bottlenecks for system performance and storage system
reliability. Even though disk performance has been improving continuously,
microprocessor performance has advanced much more rapidly. The performance of
microprocessors has improved at about 50 percent or more per year, but disk access
times have improved at a rate of about 10 percent per year and disk transfer rates at a
rate of about 20percent per year. In addition, since disks contain mechanical elements,
they have much higher failure rates than electronic parts of a computer system. If a
disk fails, all the data stored on it is lost.
A disk array is an arrangement of several disks, organized so as to increase
performance and improve reliability of the resulting storage system. Performance is
increased through data striping.
Data striping distributes data over several disks to give the impression of having a
single large, very fast disk. Reliability is improved through redundancy. Instead of
having a single copy of the data, redundant information is maintained. The redundant
information is carefully organized so that in case of a disk failure, it can be used to
reconstruct the contents of the failed disk.

2. Disk arrays that implement a combination of data striping and redundancy are called
redundant arrays of independent disks, or in short, RAID.1 Several RAID organizations,
referred to as RAID levels, have been proposed. Each RAID level represents a different
trade-off between reliability and performance. In the remainder of this section, we will
first discuss data striping and redundancy and then introduce the RAID levels that have
become industry standards.
While having more disks increases storage system performance, it also lowers overall
storage system reliability. Assume that the mean-time-to-failure, or MTTF, of a single
disk is 50; 000 hours (about 5:7 years). Then, the MTTF of an array of 100 disks is only
50; 000=100 = 500 hours or about 21 days, assuming that failures occur independently
and that the failure probability of a disk does not change overtime. (Actually, disks
have a higher failure probability early and late in their lifetimes.
Early failures are often due to un detected manufacturing defects; late failures occur
Reliability of a disk array can be increased by storing redundant information. If a disk
failure occurs, the redundant information is used to re construct the data on the failed
disk.

3. Redundancy can immensely increase the MTTF of a disk array. When incorporating
redundancy into a disk array design, we have to make two choices. First, we have to
decide where to store the redundant information. We can either store the redundant
information on a small number of check disks or we can distribute the redundant
information uniformly over all disks. The second choice we have to make is how to
compute the redundant information .Most disk arrays store parity information: In the
parity scheme, an extra check disk contains information that can be used to recover
from failure of any one disk in the array.
Assume that we have a disk array with D disks and consider the first bit on each data
disk. Suppose that i of the D data bits are one. The first bit on the check disk is set to
one if i is odd, otherwise it is set to zero. This bit on the check disk is called the parity
of the data bits. The check disk contains parity information for each set of
corresponding D data bits.In a RAID system, the disk array is partitioned into reliability
groups, where a reliability group consists of a set of data disks and a set of check disks.
A common redundancy scheme (see box) is applied to each group. The number of
check disks depends on the RAID level chosen. In the remainder of this section, we
assume for ease of explanation that there is only one reliability group. The reader
should keep in mind that actual RAID implementations consist of several reliability
groups, and that the number of groups plays a role in the overall reliability of the

4. Most of our systems we are having one hard disk for storing the data if due to any mechanical
failure or some disk failure it may be that hard disk will get corrupt and all our data get lost. So
what could be the solution we could replace a single hard disk with multiple hard drives so what
will be the advantage of this, our data will remain secure our data for a longer time. How will
define the RAID now? RAID is nothing but redundant array of inexpensive or independent
disks. What is the advantage of using of RAID? In this system, because of failure one hard disk
data loss does not take place;
RAID is collection of various disk organization techniques.
It is used to get better disk performance, reliability or both.
RAID can be structured in many ways, there many levels of RAID such as RAID 0,1,2,34,5,6,10
Before Discussing about RAIDS first we need to about Data Striping?
Data striping is the technique of splitting the data for spreading the data across
multiple hard drives.
The division of data can be done at 3 levels.
Bit Level
Byte Level(1 byte=8bits)
Block Level(Group of number of bytes)

5. A1
B1
A2
B2
A3
B3
STRIPE
Stripe 1 Stripe 2 Stripe 3

6. RAID-0:Data Striping
BLOCK 1
BLOCK 3
BLOCK 5
BLOCK 7
BLOCK 2
BLOCK 4
BLOCK 6
BLOCK 8
RAID-0
DRIVE 1 DRIVE 2
Block data Striping
Fault Tolerance=None(means of our hard disk fails we lost the data)
Performance: Fast
Storage Space=100% (consider both hard drives have 50 GB each that could be
used to 100 GB utilized to storing the data.

7. RAID-1:Mirroring
BLOCK 1
BLOCK 2
BLOCK 3
BLOCK 4
BLOCK 1
BLOCK 2
BLOCK 3
BLOCK 4
RAID-1
DRIVE 1 DRIVE 2
It uses the concept of mirroring; what is mirroring? Means is written identically
to two drives(mirror set).
Required at least 2 disks
Advantage:
Fault Tolerance: Mirrored Set(it means if any one hard disk failed we can replace
with new hard disk in place of old one, here we can’t lost the data even one hard
disk failed.
Disadvantage:
Performance: 50%: Because if one disk is allocated for backup of the another disk so it
occupies extra 50% space.

8. RAID-2
A1
B1
C1
D1
Use of Mirroring as well as stores error correcting codes for its data striped on
different disks
It is bit level striping hamming ECC
Each data bit in a word is recorded on a separated disk and ECC codes of the data
words are stored on a set disks
Due to its complex structure and high cost RAID 2 not commercially available
A2
B2
C2
D3
A3
B3
C3
D3
A0
B0
C0
D0
ECC/Ax
ECC/Bx
ECC/Cx
ECC/Dx
ECC/Ay
ECC/By
ECC/Cy
ECC/Dy
ECC/Az
ECC/Bz
ECC/Cz
ECC/Dz

9. Byte level striping along the parity is used at this level.
It uses an extra disk for storing all the parity information, if any drive gets failed, the
parity restores the failed disk
If parity drive fails fault tolerance is not possible.
Data Disks
Parity Disk
B1 B2 B3
B4
B7
B5
B8
B6
B9
p1
p2
p3

10. RAID-4:Block Striping with parity
A1
B1
C1
D1
A2
B2
C2
D2
Block level striping along with parity is used at this level
It uses one extra disk for storing all the parity information
A3
B3
C3
D3
Ap
Bp
Cp
Dp
If any drive gets failed, the parity drive restores the failed disk
If parity disk fails fault tolerance is not possible

11. Block level striping along with parity is used at this level
It requires at least 3 hard disks
Fault Tolerance: Parity(You can handle one disk failure)
Performance: Fast
Storage Space: 75%(Loose one hard disk space)
It can handle fault tolerance at two disk failures
DISK 1
A
B
P3
DISK 2
C
P2
D
DISK 3
P1
E
F

12. It is enhanced version of RAID level 5
In this Level for each block double parity are distributed among several hard disks
so this particular level helps to restore failure of two hard drives also successfully
restore which was not possible in RAID 5
This RAID level 6 is mostly utilized in big companies such as google, face book so on
DISK 1
A
C
P5
DISK 2
B
P3
P6
DISK 3
P1
P4
E
DISK 3
P2
D
F

13. The data blocks are spread over separate drives and mirrored (duplicated)this arrangement
provides both speed and fault tolerance.
This is the recommended RAID configuration for most database installations(if cost is not an
issue)

14. If data loss is not an issue, RAID Level 0 improves overall system performance at the
lowest cost. RAID Level 0+1 is superior to RAID Level 1. The main application areas for
RAID Level 0+1 systems are small storage subsystems where the cost of mirroring is
moderate. Sometimes RAID Level 0+1 is used for applications that have a high
percentage of writes in their workload, since RAID Level 0+1 provides the best write
performance. RAID levels 2 and 4 are always inferior to RAID levels 3 and 5,
respectively. RAID Level 3 is appropriate for workloads consisting mainly of large
transfer requests of several contiguous blocks. The performance of a RAID Level 3
system is bad for workloads