CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [167]
Figure 11-30 IDs don’t conflict between separate SCSI chains.
Termination
Whenever you send a signal down a wire, some of that signal reflects back up the wire, creating an echo and causing electronic chaos. SCSI chains use termination to prevent this problem. Termination simply means putting something on the ends of the wire to prevent this echo. Terminators are usually pull-down resistors and can manifest themselves in many different ways. Most of the devices within a PC have the appropriate termination built in. On other devices, including SCSI chains and some network cables, you have to set termination during installation.
The rule with SCSI is that you must terminate only the ends of the SCSI chain. You have to terminate the ends of the cable, which usually means that you need to terminate the two devices at the ends of the cable. Do not terminate devices that are not on the ends of the cable. Figure 11-31 shows some examples of where to terminate SCSI devices.
Because any SCSI device might be on the end of a chain, most manufacturers build SCSI devices that can self-terminate. Some devices can detect that they are on the end of the SCSI chain and automatically terminate themselves. Most devices, however, require you to set a jumper or switch to enable termination (Figure 11-32).
Figure 11-31 Location of the terminated devices
Figure 11-32 Setting termination
Protecting Data with RAID
Ask experienced techs “What is the most expensive part of a PC?” and they’ll all answer in the same way: “It’s the data.” You can replace any single part of your PC for a few hundred dollars at most, but if you lose critical data—well, let’s just say I know of two small companies that went out of business just because they lost a hard drive full of data.
Data is king; data is your PC’s raison d’être. Losing data is a bad thing, so you need some method to prevent data loss. Of course, you can do backups, but if a hard drive dies, you have to shut down the computer, reinstall a new hard drive, reinstall the operating system, and then restore the backup. There’s nothing wrong with this as long as you can afford the time and cost of shutting down the system.
A better solution, though, would save your data if a hard drive died and enable you to continue working throughout the process. This is possible if you stop relying on a single hard drive and instead use two or more drives to store your data. Sounds good, but how do you do this? Well, first of all, you could install some fancy hard drive controller that reads and writes data to two hard drives simultaneously (Figure 11-33). The data on each drive would always be identical. One drive would be the primary drive and the other drive, called the mirror drive, would not be used unless the primary drive failed. This process of reading and writing data at the same time to two drives is called disk mirroring.
Figure 11-33 Mirrored drives
If you really want to make data safe, you can use a separate controller for each drive. With two drives, each on a separate controller, the system will continue to operate even if the primary drive’s controller stops working. This super-drive mirroring technique is called disk duplexing (Figure 11-34). Disk duplexing is also much faster than disk mirroring because one controller does not write each piece of data twice.
Figure 11-34 Duplexing drives
Even though duplexing is faster than mirroring, they both are slower than the classic one-drive, one-controller setup. You can use multiple drives to increase your hard drive access speed. Disk striping (without parity) means spreading the data among multiple (at least two) drives. Disk striping by itself provides no redundancy. If you save a small Microsoft Word file, for example, the file is split