HP A7143A RAID160 SA Controller Support Guide

RAID Technology Overview

Logical Drives and Arrays

Chapter 1 17

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 spread evenly over all of the physical disks in a

logical drive (see Figure 1-3).

Figure 1-3 Data Striping (S1-S4) of Data Blocks B1-B12

For data in the logical drive to be readable, the data block sequence must

be the same in every stripe. This sequencing process is performed by the

array controller (HP RAID160 SA), which sends the data blocks to the

disk drive write heads in the correct order.

A natural consequence of the striping process is that each physical disk

in a given logical drive will contain the same amount of data. If one

physical disk has a larger capacity than other physical disks in the same

logical drive, the extra capacity is wasted because it cannot be used by

the logical drive.

The group of physical disks containing the logical drive is called a drive

array (or just array). Since all of the physical disks in an array are

commonly configured into just one logical drive, the term array is also

often used as a synonym for logical drive.

Each logical drive is distributed over all of the physical disks within an

array. A logical drive can also extend over more than one channel on the

same controller, but it cannot extend over more than one controller.

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