UNIX System Administration Handbook - Evi Nemeth [81]
• After you have added a new SCSI device, check the listing of devices discovered by the OS when it reboots to make sure that everything you expect is there. Most SCSI drivers will not detect multiple devices that have the same SCSI address, which is an illegal configuration. SCSI address conflicts can lead to very strange behavior.
• Some expansion boxes (enclosures with a power supply and one or more SCSI devices) terminate the bus inside the box. If devices are attached after the expansion box, you can have reliability problems with any of the devices on the SCSI chain. Always double-check that you have exactly two terminators and that they are both at the ends of the bus.
• The thumbwheel used to set a device’s SCSI address is sometimes connected backwards. When this happens, the thumbwheel will change the SCSI address, but not to the displayed value.
• When figuring the length of your SCSI-2 bus, make sure you count the cables inside devices and expansion boxes. They can be quite long. Also remember that the maximum length can be reduced if older SCSI devices are added to a newer SCSI bus.
• Never forget that your SCSI controller uses one of the SCSI addresses!
The IDE interface
IDE, also called ATA (for AT Attachment), was designed to be simple and inexpensive. It is most often found on PCs or low-cost workstations. The controller is built into the disk, which reduces interface costs and simplifies the firmware. IDE became popular in the late 1980s. Shortly thereafter, ATA-2 was developed to satisfy the increasing demands of consumers and hard drive vendors.
ATA-2 adds faster Programmed I/O (PIO) and Direct Memory Access (DMA) modes and extends the bus’s Plug and Play features. It also adds a feature called Logical Block Addressing (LBA), which (in combination with an enhanced PC BIOS) overcomes a problem that prevented BIOSes from accessing more than the first 1024 cylinders of a disk. This constraint formerly limited disk sizes to 504MB. Who would have thought a disk could get that big!
Since the BIOS manages part of the bootstrapping process, it is sometimes necessary to create a small bootable partition within the first 1024 cylinders to ensure that the kernel can be loaded by an old BIOS. Once the kernel is up and running, the BIOS is not needed and you can access the rest of your disk. This silly maneuver is unnecessary on modern hardware since LBA gets rid of cylinder-head-sector (CHS) addressing in favor of a linear addressing scheme.
ATA-3 adds additional reliability, more sophisticated power management, and self-monitoring capabilities. ATA-4 is still being developed. Ultra-ATA is an attempt to bridge the gap between ATA-3 and ATA-4, adding high-performance modes, called Ultra DMA/33 and Ultra DMA/66, that extend the bus bandwidth from 16 MB/s to 33 MB/s and 66 MB/s, respectively. ATA-4 is also a much-needed attempt to merge ATA-3 with the ATA Packet Interface (ATAPI), a protocol that allows CD-ROM and tape drives to work on an IDE bus.
IDE disks are almost always used internally (unless you consider a disk hanging out the side of the computer for testing purposes “external”). The maximum cable length for an ATA-2 bus is a mere 18 inches, which can make it difficult even to reach your system’s top drive bay. In addition to the short cable length, an IDE bus can accommodate only two devices. To compensate for these shortcomings, most manufacturers provide more than one IDE bus on their motherboards.
IDE devices are accessed in a connected manner, which means that only one device can be active at a time. Therefore, performance is best if you spread the devices out over multiple buses. Put fast devices such as hard drives on one bus and tapes or CD-ROMs on another to prevent the slower devices from hindering the faster ones. SCSI handles multiple devices on a bus much better than does IDE.4
The IDE connector is a 40-pin header that connects the drive to