Managing RAID on Linux - Derek Vadala [27]
Shortly afterward, low-voltage differential (LVD) devices were introduced. LVD devices provided an increased maximum throughput like HVD, but limited the overall cable length to 12 meters. However, LVD also dramatically decreased hardware costs when compared to HVD. By lowering the voltage of the SCSI bus, LVD allowed a single chip to control both the SCSI devices and the voltage. This decrease in voltage also allowed LVD and SE to coexist on the same bus. LVD is now the standard and is supported by all recent SCSI devices.
SCSI Versus ATA
Overall, SCSI is a much better choice than ATA, both as a standalone and as part of an array. It allows more devices per channel and provides higher throughput. It also has a much larger command set, compared to ATA, which translates into better performance and increased reliability. The only major drawback of SCSI is price. SCSI drives and controllers are generally more expensive, with SCSI drives typically costing two or three times as much per megabyte as their ATA equivalents. (Although today, some mid-range motherboards are available with built-in SCSI controllers at little extra cost.) If you plan to use external SCSI devices, you will need to spend extra money on cabling and external disk enclosures. On the other hand, ATA does not support external devices at all, so its expandability is limited.
Speed
In the past, SCSI outperformed ATA by leaps and bounds, but ATA has caught up substantially in recent years. Today, ATA disks perform as well as SCSI disks, so speed isn't as much of a factor as it was just three or four years ago. But, with SCSI, you can populate an I/O channel with enough devices to fully utilize the entire pipe. With ATA, you are really limited to one device per channel if you want decent performance from that device, and that's not enough to utilize the full pipe when working with the most recent ATA specifications.
High-end SCSI drives have data throughputs of about 40 MB/s. When using Ultra 160 SCSI, you would need three or four drives on a single chain to take full advantage of your bandwidth. ATA drives operate at much slower speeds, so if you were using Ultra ATA/100, you could not possibly populate a single channel with enough drives to take full advantage of your I/O pipe, even if you put two devices on the same channel. The Choosing Hard Drives section, later in this chapter, discusses hard disk bottlenecks in more detail.
Configuration
Many people complain about the complexities and pitfalls of SCSI termination. But it's really quite simple. The beginning and end of every SCSI chain must be terminated. Figure 2-21 illustrates termination on a controller to which only internal devices are connected. The controller card is usually the last device on a channel and comes with built-in termination enabled.
Figure 2-21. Modern controller cards provide onboard termination.
If you plan to use both internal and external devices on the same chain, then you will need to terminate the external portion of the chain. Figure 2-22 shows how to terminate a controller with both internal and external devices. Depending on your controller, you might also need to disable the controller's termination in the SCSI BIOS, although many cards automatically do this once devices are connected to the external connector.
Figure 2-22. The last device on an external SCSI chain must be terminated.
As shown in Figure 2-23, the same methodology applies if you are using only external devices.
Figure 2-23. When using only external devices, the last disk on the chain and the SCSI card are terminated.
Finally, specifications dictate that any unused connections on an internal cable appear after the last SCSI device on that chain. In practice, this recommendation is often ignored, and many users report no errors when breaking this rule. I have never had problems using cables with more connectors than drives