Disk Duplexing

Disk Duplexing

In the event of disk channel failure (by a controller card or cable), access to all data on the channel stops and a message appears on the file server console screen (if your users don’t let you know about it first). Even though drives can be mirrored, all disk activity on the mirrored pair ceases if the mirrored drives are connected to the same disk controller.

Disk duplexing performs the function of simultaneously writing data to disks located on different channels. As Figure 9.7 illustrates, each hard disk in a duplexed pair connects to a separate hard disk controller. This figure shows a configuration in which the drives are housed in separate disk subsystems. Each subsystem also has a separate power supply. Disk duplexing offers a more reliable setup than is possible with mirroring because a failure of one disk drive’s power supply doesn’t disable the server. Instead, the server continues to work with the system that remains under power.

Working on the same channel is analogous to going to a baseball game when only one gate into the stadium is open. You can enter or exit through only one gate (channel) at the stadium (file server), and the crowd (data) can get backed up on both sides. If more than one gate (another channel) is open, though, the crowd (data) doesn’t become backed up on both sides of the fence (file server or workstation).

Duplexing protects information at the hardware level with duplicate channels (controller cards and cables) and duplicate hard drives (refer to Figure 9.7).

Mirroring uses one controller card and two hard drives (refer to Figure 9.4). The point of failure for this setup is primarily the controller card or the cable connecting the drives to the controller card. Disk duplexing uses two controller cards and a minimum of one drive per controller card. The point of failure is reduced with duplicate hardware.

A number of different vendors also offer RAID protection at the hardware level on their server products. This protection is independent of the operating system, so if you really feel that RAID 5 on your Windows 95 workstation is a necessity, these software vendors might have a solution for you. Third-party products also permit the concept of redundancy to be taken to its logical extreme, resulting in the mirroring of entire Windows NT Server machines. This mirroring protects against the failure of virtually any single piece of hardware you can imagine, from a memory stick to a motherboard. Remember, though, that duplicate servers can get a bit expensive, so they are not recommended for everyone.

The previous sections examined a number of different disk configurations. Exercise 9.1 shows how you implement these RAID levels and other disk configuration options in Windows NT. The primary program for managing disk storage resources is the Disk Administrator, a tool that is usable only by members of the Administrators or Server Operators groups.