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FIGURE 9.1 Disk mirroring

The drives do not need to be identical, but this helps. Both drives must have the same amount of free space to allow a mirror to be formed. For example, you have two 4GB drives; one has 3GB free, and the other has 2GB free. You can create one 2GB mirrored system.

Mirroring is an implementation of RAID level 1, which is discussed in detail later in this chapter.

Disk Duplexing

As with mirroring, duplexing also saves data to a mirror drive. In fact, the only major difference between duplexing and mirroring is that duplexing uses two separate disk controllers (one for each disk). Thus, duplexing provides not only a redundant disk, but a redundant controller and data ribbon as well. Duplexing provides fault tolerance even if one of the controllers fails. Figure 9.2 shows a duplexed disk system. Compare this with Figure 9.1. Notice that there is now an extra disk controller in the system.

Duplexing is also an implementation of RAID level 1.


Disk duplexing

Disk Striping

From a performance point of view, writing data to a single drive is slow. When three drives are configured as a single volume, information must fill the first drive before it can go to the second and fill the second before filling the third. If you configure that volume to use disk striping, you will see a definite performance gain. Disk striping breaks up the data to be saved to disk into small portions and sequentially writes the portions to all disks simultaneously in small areas called stripes. These stripes maximize performance because all of the read/write heads are working constantly. Figure 9.3 shows an example of striping data across multiple disks. Notice that the data is broken into sections and that each section is sequentially written to a separate disk.

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