Picking the Correct Hardware

When you pick an interface, matching the card to the disks and other devices it will control is important. Both ATA and SCSI come in several different levels, each of which provides better performance than earlier levels. In theory, new interfaces can manage older devices; however, in practice complications sometimes arise. Older devices may degrade the performance of newer devices attached to the same bus, particularly in the case of ATA devices. Differing termination and bus width requirements can make mixing SCSI devices of radically different ages a real nightmare.

Picking the Correct ATA Hardware

ATA hardware is known by several names and comes with varying capabilities. Many names are largely marketing hype or unofficial names assigned to indicate the device's maximum transfer speed. Table 2.1 summarizes the names and basic characteristics of these devices. Note that the Added PIO Modes and Added DMA Modes columns are cumulative; later standards support all of the modes of earlier standards, plus the modes listed in their entries in these columns. Unlike SCSI's 50-and 68-pin cables, ATA's 40- and 80-wire cables are physically compatible, so you can use a new cable to attach an old device to a new or old controller.

Note At the time of this writing, an ATA-7 standard is under discussion, but it has not yet been finalized. It's likely to include support for 133MB/S transfers and UltraDMA mode 6. In practice, hardware often leads standards, so devices with these features are already on the market.

Table 2.1 : ATA Hardware Types

Official Name

Unofficial Names

Maximum Speed

Added PIO Modes

Added DMA Modes

Cable Type

ATA-1

IDE

11.1MB/S

0, 1,2

0, 1,2, Multiword 0

40-wire

ATA-2

EIDE

16.6MB/S

3,4

Multiword 1, Multiword 2

40-wire

ATA-3

16.6MB/S

40-wire

ATA-4

UltraDMA/33, ATA/33

33.3MB/S

UltraDMA 0, UltraDMA 1, UltraDMA 2

40-wire or 80-wire

ATA-5

UltraDMA/66, ATA/66

66.6MB/S

UltraDMA 3, UltraDMA 4

80-wire

ATA-6

UltraDMA/100, ATA/100

100MB/S

UltraDMA 5

80-wire

In addition to the standards outlined in Table 2.1, many ATA devices support the ATA

In addition to the standards outlined in Table 2.1, many ATA devices support the ATA

Packet Interface (ATAPI), which is a set of software extensions to support CD-ROM drives, tape drives, removable disks, and other non-hard-disk devices. ATAPI is a matter of drive and software communication, though; the ATA adapter doesn't need explicit ATAPI support.

When considering an upgrade, check your existing devices' documentation to learn which speeds they support. You can buy a more capable ATA controller than you need now, but try to put devices of similar levels on the same bus whenever possible. For instance, if your motherboard supports 33MB/S transfers and you buy an ATA-6 (100MB/s) card to supplement it, put slow devices on the motherboard and fast ones on the expansion card.

Some very high-end ATA controllers support Redundant Array of Independent Disk (RAID) technology. RAID enables you to link together multiple hard disks for improved speed, improved reliability, or both. RAID controllers tend to be rather expensive, but they're well worth the cost for high-end applications such as busy servers. They may require special Linux drivers, though, so check on driver availability, as described in Chapter 1, before buying a RAID controller. Also, the Linux kernel supports software RAID, which is adequate for many purposes.

Before you buy an ATA controller, check on Linux driver availability. The Linux kernel configuration tool provides information on supported ATA controllers. These drivers should be built into the Linux kernel itself, at least if you intend to boot from a disk connected to a controller you buy.

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