7.1. Drive Array Technologies

Operating systems address a logical drive as a single, contiguous storage space, although it is made up of several physical drives. Assembling drives together in an array quickly and reliably provides access to many gigabytes of data to users.

The advantages of a drive array implementation are as follows:

• Effective high-speed data transfer rates

• Ability to handle simultaneous multiple requests

• Increased storage capacity

• Flexibility in configuring data

• High reliability

Drive array technology can access data from multiple drives faster than from any one physical drive and enables the arrayed drives to service multiple requests simultaneously.

Drives within a drive array can have different sizes. All disks will adjust their size to the capacity of the smallest disk. The excess capacity of all larger drives is not accessible.

7.1.1 Number of Disks in an Array

You can increase the I/O bandwidth of an array by adding drives. This addition allows data to be read from or written to a large number of drives simultaneously.

Many customers buy a few large-capacity drives as opposed to a larger number of low-capacity drives because of cost and management concerns and because they do not understand performance implications. Performance can suffer when there are insufficient drive spindles.

The RAID level and the number of drives affect the available I/O bandwidth for a given disk configuration. You must have enough drives (regardless of the drive capacity) to sustain the I/O rates of the application.

7.1.2 Logical Drives

A logical drive is a subset of an array that is presented to the operating system as a single drive. The operating system cannot see the individual disks and assumes there is only one disk per logical drive.

An example is shown in Figure 7-1. This graphic shows that seven physical drives could be configured as one logical drive of 127GB (top right) or as three logical drives of 42.5GB each. (Other logical drive configurations would also be possible.)

Figure 7-1. Logical drive configurations of multiple physical drives.

Figure 7-2 shows a situation where three physical drives are configured as three logical drives, and each logical drive spans a portion of the physical drive.

Figure 7-2. Logical drives spanning physical drives.

Note

Drives with different sizes, speeds, and SCSI protocols can be mixed within an array. The configuration might result in wasted space and performance degradation, but it will work.

7.1.3 Drive Array Features

Drive arrays provide the following features:

• Data striping across multiple drives A file is divided into a selected number of sectors and then written across a series of drives. The process of writing or reading a file across multiple drives is much faster than the same process on a single drive.

• Multiple channels The drive array uses multiple channels simul-taneously (depending on the controller used), thereby increasing performance.

• Request processing Because multiple commands can be issued across multiple channels, all commands can be processed at the same time. The requests are processed in the most logical order using tagged command queuing (TCQ).

7.1.3.1 TAGGED COMMAND QUEUING

TCQ allows a device to accept multiple commands from a host I/O controller and organize them for the most efficient operations on the disk drive. The host I/O controller and the device keep track of the I/O requests by using numbered tags.

TCQ offers better performance gains with randomly accessed I/O traffic. With sequentially accessed data, an implied sorting is already being applied so the performance gain will be minimum.