3. Understanding RAID
page 17
LaCie Biggest S1S
User’s Manual
3.3. RAID 5 And RAID 5 + Hot Spare
Independent Data Disks With Distributed Parity Blocks
This is the most versatile RAID level and offers high I/O transaction rates, which greatly helps applications that perform large
numbers of concurrent requests. If one drive in a RAID 5 array fails, the lost data can be rebuilt from data on the remaining,
functional disks.
Essentially, RAID level 5 is a striped set with parity, and requires at least three disks to implement. In this configuration, data is
saved across several hard disks, as in a RAID 0 array, but parity information is also saved across the disks. It is this parity
information which provides the fault-tolerance protection; if one hard disk in the set fails, the data that it contains can be
rebuilt by utilizing the parity information from the other hard disks. With the parity data being stored across the array, this
also serves to maximize the amount of storage capacity available amongst the disks in the array while still providing data
redundancy.
Storage capacity in a RAID level 5 configuration is the result of a mathematical expression that compares data from the drives
and a calculates another piece of data called parity. In this situation, then, storage capacity is calculated by multiplying the
number of disks in the array, minus one, by the capacity of the smallest disk in the array. So, for instance, if a RAID 5 array is
created with four drives of varying capacities of 40GB, 50GB, 60GB and 70GB, the total capacity of the array would be
120GB [3 (4 disks – 1 disk) x 40 = 120].
Characteristics & Advantages
• Highest Read data transaction rate
• Medium Write data transaction rate
• High efficiency through a low ratio of ECC (Parity) disks
to data
• Good aggregate transfer rate
Recommended Uses
• File and Application Servers
• Database Servers
• Web, E-mail and News Servers
• Intranet Servers
A
D
P
JKL
G
B
E
J
C
H
K
F
I
E
D
C
B
A
CONTROLLER
P
ABC
L
P
GHI
P
DEF
This diagram represents a
RAID 5 array, consisting of
four disks, which are
connected to the Controller.
Parity blocks are represented
by the letter P.
