In most cases, your motherboard's ATA controller will work just fine for handling ATA devices. For a typical setup, then, there's seldom any need to change the configuration. There are reasons you might want to add to or replace your motherboard's ATA controller, though:
Lack of Linux Driver Support All ATA controllers work to a minimal extent using an old compatibility mode that dates back to the days of the ST-506 interface. By today's standards, though, this mode is extremely slow; to get the most out of an ATA controller, you need Linux support for it. Linux includes such support for most ATA controllers, but occasionally this support may be missing or buggy. This is particularly likely to be true if you buy a motherboard that uses a brand-new chipset for which Linux ATA drivers haven't been written. In such a case, you might want to disable the built-in ATA controller and use an add-on card instead.
Too Many Disks As noted earlier, it's easy to consume all four devices that can be controlled by a typical IA-32 motherboard. In this situation, you must add a second controller to supplement, rather than replace, the motherboard's controller. A typical expansion ATA controller can handle two additional chains, for a total of eight devices. Alternatively, you could use SCSI devices for expansion and use a SCSI host adapter.
Need for SCSI Support If you've got a SCSI device that you want to use, such as a SCSI scanner or tape drive, you must add a SCSI host adapter to your system. Some SCSI devices ship with low-end SCSI host adapters that may not work under Linux, so replacing them is a necessity. Even if you don't currently have any SCSI devices, you might prefer the features of SCSI models to non-SCSI models for some nondisk devices, in which case adding a SCSI host adapter is required.
Speed Upgrade If your motherboard is more than a year or two old, it may not support the speeds that a modern hard disk can support. In such a situation, you may want to disable the motherboard's ATA controller in favor of one on a more modern controller card. You're not likely to see any benefit of such an upgrade unless you also buy a new hard disk, though; at any given point in time, the best available ATA controllers exceed the sustained data transfer speeds possible with the best ATA hard disks.
Fundamentally the same rules apply to upgrades for SCSI host adapters as for ATA controllers, although some details differ. Only the largest systems will exceed SCSI's device limit, although past half a dozen or so devices, it's often much easier to configure a second SCSI host adapter than it is to add more devices to the primary SCSI bus. You might also want two SCSI host adapters to support both recent and older SCSI devices, because it's sometimes difficult to get SCSI devices of wildly differing ages to coexist on a single SCSI bus.
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