RAID ARRAYS
Redundant Array of
Inexpensive Discs
WHAT IS RAID ARRAYS?
RAID is an acronym for
Redundant Array of
Independent Drives (or Disks),
also known as Redundant
Array of Inexpensive Drives (or
Disks)
The various types of RAID are
data storage schemes that
divide and/or replicate data
among multiple hard drives
WHY USE RAID?
Improved Reliability
Improved Performance
Fault Tolerance
Improved Availability
Higher Data Security
KEY TERMS
Mirroring - the copying of data to more than one
disk
 Striping - the splitting of data across more than
one disk
 Parity - a redundancy check that ensures that the
data is protected without having to have a full set
of duplicate drives.
 Duplexing - an extension of mirroring that is
based on the same principle as that technique
expect it goes one step further in that it also
duplicates the hardware that controls the two
hard drives (or sets of hard drives).

RAID Arrays
RAID - REDUNDANT ARRAY OF
INDEPENDENT DISKS
RAID Arrays
RAID
Controller
Host
RAID Array
RAID COMPONENTS
Physical
Array
RAID
Controller
Logical
Array
Host
RAID Array
RAID Arrays
Logical
Array
DATA ORGANIZATION: STRIPS AND
STRIPES
RAID Arrays
Stripe 1
Stripe 2
Stripe 3
Strips
RAID LEVELS
0 Striped array with no fault tolerance
 1 Disk mirroring
 3 Parallel access array with dedicated parity disk
 4 Striped array with independent disks and a
dedicated parity disk
 5 Striped array with independent disks and
distributed parity
 6 Striped array with independent disks and dual
distributed parity
 Combinations of levels (I.e., 1 + 0, 0 + 1, etc.)

RAID Arrays
RAID 0
A striped set of at least two
disks without parity
The data is broken down
into blocks and each block is
written to a separate disk
drive
Best performance is
achieved when data is
striped across multiple
controllers with only one
drive per controller
RAID 0 – STRIPED ARRAY WITH NO FAULT TOLERANCE
Block 0
4
3
2
1
RAID
Block 0
4
3
1
2
Controller
Host
RAID Arrays
- 10
ADVANTAGES OF RAID 0
I/O performance is greatly
improved by spreading
the I/O load across many
channels and drives
No parity calculation
overhead is involved
Very simple design
Easy to implement
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
RAID 1 – DISK MIRRORING
Block 0
1
RAID
Block 0
1
Controller
Host
RAID Arrays
- 13
RAID 1 ADVANTAGES
High data availability and high
I/O rate (small block size).
Improves read performance 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
Simplest RAID storage
subsystem design – easy to
maintain
RAID 1 DISADVANTAGES
Expensive due to the extra
capacity required to
duplicate data. Overhead
cost equals 100%, while
usable storage capacity is
50%.
May not support hot swap of
failed disk when
implemented with software.
Use hardware
implementation.
RAID 0+1 – STRIPING AND MIRRORING
RAID Arrays
Block 0
3
2
1
Host
RAID
Block 0
3
2
1
Controller
RAID 1+0 – MIRRORING AND STRIPING
RAID Arrays
Block 0
3
2
1
Host
RAID
Block 0
3
2
1
Controller
RAID 0+1 VS. RAID 1+0
Benefits are identical under normal operations
 Rebuild operations are very different


RAID is 0+1 is a poorer solution and is less common
RAID Arrays
RAID 1+0 uses a mirrored pair – only 1 disk is
rebuilt if a disk fails
 RAID 0+1 if a single drive fails, the entire stripe is
faulted

RAID REDUNDANCY: PARITY
0
4
8
RAID
Controller
RAID Arrays
1
5
9
2
6
10
3
7
11
Host
0123
4567
8 9 10 11 - 19
Parity Disk
PARITY CALCULATION
5 + 3 + 4 + 2 = 14
Data
3
Data
4
Data
2
Data
5 + 3 + ? + 2 = 14
? = 14 – 5 – 3 – 2
?=4
14
Parity
RAID Array
RAID Arrays
The middle drive fails:
5
RAID 3 – PARALLEL TRANSFER WITH
DEDICATED PARITY DISK
RAID Arrays
Block 0
3
2
1
Host
RAID0
Block
Controller
Block
Parity1
Generated
Block 2
Block 3
P0123
RAID 4 – STRIPING WITH DEDICATED
PARITY DISK
Block 0
Block 4
Block 5
Block 0
Parity
RAID0
Block
Generated
Controller
Block 2
Block 6
P0123
Block 3
Host
Block 7
P0123
P4567
RAID Arrays
Block 1
RAID 5 – INDEPENDENT DISKS WITH
DISTRIBUTED PARITY
Block 0
Block 4
Block 5
Block 0
4
Parity
RAID4
Block
0
Generated
Controller
Block 2
Block 6
P4
05
16
27
3
Block 3
Host
P4567
P0123
Block 7
RAID Arrays
Block 1
RAID 6 – DUAL PARITY RAID
Two disk failures in a RAID set leads to data
unavailability and data loss in single-parity
schemes, such as RAID-3, 4, and 5
 Increasing number of drives in an array and
increasing drive capacity leads to a higher
probability of two disks failing in a RAID set
 RAID-6 protects against two disk failures by
maintaining two parities


Even-Odd, and Reed-Solomon are two commonly
used algorithms for calculating parity in RAID-6
RAID Arrays
Horizontal parity which is the same as RAID-5 parity
 Diagonal parity is calculated by taking diagonal sets
of data blocks from the RAID set members

RAID IMPLEMENTATIONS

Hardware (usually a specialized disk controller card)





Software





Generally runs as part of the operating system
Volume management performed by the server
Provides more flexibility for hardware, which can reduce
the cost
Performance is dependent on CPU load
Has limited functionality
RAID Arrays

Controls all drives attached to it
Performs all RAID-related functions, including volume
management
Array(s) appear to the host operating system as a regular
disk drive
Dedicated cache to improve performance
Generally provides some type of administrative software
HOT SPARES
RAID Arrays
RAID
Controller
HOT SWAP
RAID
Controller
RAID Arrays
RAID
Controller

CHECK YOUR KNOWLEDGE
What is a RAID array?
 What benefits do RAID arrays provide?
 What methods can be used to provide higher data
availability in a RAID array?
 What is the primary difference between RAID 3
and RAID 5?
 What is a hot spare?

RAID Arrays