Contributed by Satoshi Asami <asami@FreeBSD.org>.
17 February 1998.
As mentioned in the SCSI section, virtually all SCSI hard drives sold today are SCSI-2 compliant and thus will work fine as long as you connect them to a supported SCSI host adapter. Most problems people encounter are either due to badly designed cabling (cable too long, star topology, etc.), insufficient termination, or defective parts. Please refer to the SCSI section first if your SCSI hard drive is not working. However, there are a couple of things you may want to take into account before you purchase SCSI hard drives for your system.
Rotational speeds of SCSI drives sold today range from around 4,500RPM to 15,000RPM. Most of them are either 7,200RPM or 10,000RPM, with 15,000RPM becoming affordable (June 2002). Even though the 10,000RPM drives can generally transfer data faster, they run considerably hotter than their 7,200RPM counterparts. A large fraction of today's disk drive malfunctions are heat-related. If you do not have very good cooling in your PC case, you may want to stick with 7,200RPM or slower drives.
Note that newer drives, with higher areal recording densities, can deliver much more bits per rotation than older ones. Today's top-of-line 7,200RPM drives can sustain a throughput comparable to 10,000RPM drives of one or two model generations ago. The number to find on the spec sheet for bandwidth is “internal data (or transfer) rate”. It is usually in megabits/sec so divide it by 8 and you will get the rough approximation of how much megabytes/sec you can get out of the drive.
(If you are a speed maniac and want a 15,000RPM drive for your cute little PC, be my guest; however, those drives become extremely hot. Do not even think about it if you do not have a fan blowing air directly at the drive or a properly ventilated disk enclosure.)
Obviously, the latest 15,000RPM drives and 10,000RPM drives can deliver more data than the latest 7,200RPM drives, so if absolute bandwidth is the necessity for your applications, you have little choice but to get the faster drives. Also, if you need low latency, faster drives are better; not only do they usually have lower average seek times, but also the rotational delay is one place where slow-spinning drives can never beat a faster one. (The average rotational latency is half the time it takes to rotate the drive once; thus, it is 2 milliseconds for 15,000RPM, 3ms for 10,000RPM drives, 4.2ms for 7,200RPM drives and 5.6ms for 5,400RPM drives.) Latency is seek time plus rotational delay. Make sure you understand whether you need low latency or more accesses per second, though; in the latter case (e.g., news servers), it may not be optimal to purchase one big fast drive. You can achieve similar or even better results by using the ccd (concatenated disk) driver to create a striped disk array out of multiple slower drives for comparable overall cost.
Make sure you have adequate air flow around the drive, especially if you are going to use a fast-spinning drive. You generally need at least 1/2” (1.25cm) of spacing above and below a drive. Understand how the air flows through your PC case. Most cases have the power supply suck the air out of the back. See where the air flows in, and put the drive where it will have the largest volume of cool air flowing around it. You may need to seal some unwanted holes or add a new fan for effective cooling.
Another consideration is noise. Many 10,000 or faster drives generate a high-pitched whine which is quite unpleasant to most people. That, plus the extra fans often required for cooling, may make 10,000 or faster drives unsuitable for some office and home environments.
Most SCSI drives sold today are of 3.5” form factor. They come in two different heights; 1.6” (“half-height”) or 1” (“low-profile”). The half-height drive is the same height as a CDROM drive. However, do not forget the spacing rule mentioned in the previous section. If you have three standard 3.5” drive bays, you will not be able to put three half-height drives in there (without frying them, that is).
The majority of SCSI hard drives sold today are Ultra, Ultra-wide, or Ultra160 SCSI. As of this writing (June 2002), the first Ultra320 host adapters and devices become available. The maximum bandwidth of Ultra SCSI is 20MB/sec, and Ultra-wide SCSI is 40MB/sec. Ultra160 can transfer 160MB/sec and Ultra320 can transfer 320MB/sec. There is no difference in max cable length between Ultra and Ultra-wide; however, the more devices you have on the same bus, the sooner you will start having bus integrity problems. Unless you have a well-designed disk enclosure, it is not easy to make more than 5 or 6 Ultra SCSI drives work on a single bus.
On the other hand, if you need to connect many drives, going for Fast-wide SCSI may not be a bad idea. That will have the same max bandwidth as Ultra (narrow) SCSI, while electronically it is much easier to get it “right”. My advice would be: if you want to connect many disks, get wide or Ultra160 SCSI drives; they usually cost a little more but it may save you down the road. (Besides, if you can not afford the cost difference, you should not be building a disk array.)
There are two variant of wide SCSI drives; 68-pin and 80-pin SCA (Single Connector Attach). The SCA drives do not have a separate 4-pin power connector, and also read the SCSI ID settings through the 80-pin connector. If you are really serious about building a large storage system, get SCA drives and a good SCA enclosure (dual power supply with at least one extra fan). They are more electronically sound than 68-pin counterparts because there is no “stub” of the SCSI bus inside the disk canister as in arrays built from 68-pin drives. They are easier to install too (you just need to screw the drive in the canister, instead of trying to squeeze in your fingers in a tight place to hook up all the little cables (like the SCSI ID and disk activity LED lines).