All modern IA-32 motherboards come with at least two built-in ATA controllers. Therefore, using an ATA disk is just a matter of setting a jumper or two, connecting the disk to the motherboard via an ATA cable, and attaching a power connector to the hard disk. Because most motherboards have only two ATA controllers, and each controller can handle just two drives, most motherboards can support only four ATA devices. Most computers have a hard disk and a CD-ROM drive. Adding a Zip or other removable-media drive and a tape backup drive leaves no room for expansion. In order to move beyond the four-drive limit, you must add a separate ATA controller, as described in the next section, "Controllers and Host Adapters." In this situation, SCSI offers the advantage of supporting more devices per chain, and hence per IRQ.
SCSI's multitasking nature is potentially important on large servers with many hard disks that are accessed simultaneously. As with resource use, though, these benefits are most important when you have many disks. When you have only one hard disk, or you have disks on separate chains, SCSI's advantages are slim or nonexistent.
Overall, SCSI devices have traditionally been good choices for high-end servers and workstations, but not for typical desktop productivity systems or small servers. ATA has been slowly eroding SCSI's advantages over the past decade, so today even most mid-sized servers can do quite nicely with ATA drives. Today, SCSI is most worthwhile if you need hundreds of gigabytes of disk space spread over several disks. Such a configuration is likely to be quite expensive. You might also consider using a low-end SCSI host adapter to drive nondisk SCSI devices, such as a tape backup unit or CD-R drive, particularly if you've run out of ATA positions.
Note To implement some functions, Linux provides an ATA-to-SCSI driver, which provides SCSI device interfaces that link to ATA devices. This feature is used to support ATA CD-R drives, for instance. You don't need an actual SCSI drive or SCSI host adapter to use this feature.
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