Managing RAID on Linux - Derek Vadala [32]
Connectors
ATA cables use a standard 40-wire, 40-pin ribbon cable, while Ultra ATA (speeds of 33 MB/s and above) uses a 40-pin, 80-wire cable (as shown in Figure 2-26). The connectors and cables might look identical, but you must use the 80-wire with Ultra ATA disks. Be sure to check the specifications when purchasing cables.
Figure 2-26. ATA cables all have the same 40-pin connectors, but Ultra AT (speeds greater than 33 MB/s) require newer 80-wire cables.
SCSI cables are much more confusing because SCSI cables have undergone more transformations than ATA cables. In most cases, you will be using a 68-pin ribbon cable for internal devices. Just make sure it's rated for the bandwidth you're using. Older external connectors have some variation (see Figure 2-27), but in most cases, 68-pin high-density (HD) connectors are used. However, newer 68-pin very high-density (VHD) connectors are making their way into the market. Decreasing the size of external connectors has made it easier for SCSI controller manufacturers to house multiple channels on a single card.
If you have different connectors on your controller card and your case, it's easy to find cables that can accommodate you. Check out http://www.scsipro.com for custom SCSI cables.
Figure 2-27. Most SCSI controllers use 68-pin high-density connectors for external connections. You may encounter some older connectors as well.
Single connector attachment (SCA)
To facilitate hot-swap disks, IBM introduced the single connector attachment (SCA) for SCSI hard disks. SCA integrates data transfer, power, and configurable options (such as SCSI ID) on a single 80-pin connector, as shown in Figure 2-28. Drives are plugged into an SCA backplane that is then connected to the SCSI bus (usually via SCSI ribbon cable) and the power supply. SCA drives are mounted in trays that slide into the backplane and lock into place, leaving the other side of the disk tray accessible from the outside of the case. These features make it easy to swap disks by eliminating the need to power down the system and dismantle the case.
Figure 2-28. SCA disks use an 80-pin connector that facilitates data transfer, power, and configuration parameters.
Since its inception, SCA has been adopted by several manufacturers, and SCA-2 is the current implementation standard. SCA drives tend to be slightly more expensive than standard drives. Cases with SCA backplanes also run on the expensive side, but SCA is a necessity for any system that needs hot-swap capability because SCA is designed to allow power-on disk swapping. Recently, SCA chassis that fit into standard desktop cases have surfaced. Enlight Corporation (http://www.enlightcorp.com) manufactures a module that fits into three 5.25″ drive bays. It supports up to five SCA disks and connects to an internal SCSI controller. Rackmount case makers also tend to sell custom drive cases that come equipped with an SCA backplane.
Individual adapter modules that allow the use of a single SCA disk with standard 68-pin SCSI cabling and power supply connection are also readily available. I've had mixed results using them, ranging from problem-free performance to SCSI channels running at less than optimal speeds. You will probably also have mixed results, but they do offer a pretty cheap way to get SCA functionality, especially on systems with only a couple of disks. Most cases support a minimum of five or six disks and are very pricey.
Power
Finally, make sure you have an adequate power supply