HP A7143A RAID160 SA Controller Support Guide, February 2007

RAID Technology Overview
Logical Drives and Arrays
Chapter 1
14
Logical Drives and Arrays
The group of physical disks containing the logical drive is called an array (or drive array). Since all the
physical disks in an array are commonly configured into a single logical drive, the term array is also used as a
synonym for logical drive.
Without an array controller, connecting extra disk drives to a system increases the total storage capacity.
However, it has no effect on the efficiency of read/write operations because data can only be transferred to one
physical disk at a time (see Figure 1-1).
Figure 1-1 Disk Drives Added to System
With an array controller, connecting extra disk drives to a system increases both the total storage capacity
and the read/write efficiency. The capacity of several physical disks is combined into one or more virtual units
called logical drives (also called logical volumes).
The read/write heads of all of the physical disks in a logical drive are active simultaneously, improving I/O
performance and reducing the total time required for data transfer (see Figure 1-2).
Figure 1-2 Physical Disks Configured into a Logical Drive (L1)
Because the read/write heads for each physical disk are active simultaneously, the same amount of data is
written to each disk during any given time interval. Each unit of data is called a block. The blocks form a set
of data stripes that are spread evenly over all the physical disks in a logical drive (see Figure 1-3).
R/W
P1 P2 P3
L1
P1 P2 P3