123 lines
6.2 KiB
Text
123 lines
6.2 KiB
Text
<!-- $Id: scsihd.sgml,v 1.2 1998-02-19 06:58:33 asami Exp $ -->
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<!-- The FreeBSD Documentation Project -->
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<!DOCTYPE chapt PUBLIC "-//FreeBSD//DTD linuxdoc//EN"> -->
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<sect2><heading> SCSI hard drives</heading>
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<p><em>Contributed by &a.asami;.<newline>17 February 1998.</em></p>
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<p>As mentioned in the <ref id="scsi" name="SCSI"> section,
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virtually all SCSI hard drives sold today are SCSI-2 compliant
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and thus will work fine as long as you connect them to a
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supported SCSI host adapter. Most problems people encounter
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are either due to badly designed cabling (cable too long,
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star topology, etc.), insufficient termination, or defective
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parts. Please refer to the <ref id="scsi" name="SCSI">
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section first if your SCSI hard drive is not working.
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However, there are a couple of things you may want to take
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into account before you purchase SCSI hard drives for your
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system.
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<sect3><heading> Rotational speed</heading>
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<p>Rotational speeds of SCSI drives sold today range from around
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4,500RPM to 10,000RPM. Most of them are either 5,400RPM or
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7,200RPM. Even though the 7,200RPM drives can generally
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transfer data faster, they run considerably hotter than their
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5,400RPM counterparts. A large fraction of today's disk drive
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malfunctions are heat-related. If you do not have very good
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cooling in your PC case, you may want to stick with 5,400RPM
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or slower drives.
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<p>Note that newer drives, with higher areal recording
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densities, can deliver much more bits per rotation than older
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ones. Today's top-of-line 5,400RPM drives can sustain a
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throughput comparable to 7,200RPM drives of one or two model
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generations ago. The number to find on the spec sheet for
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bandwidth is "internal data (or transfer) rate". It is
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usually in megabits/sec so divide it by 8 and you'll get the
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rough approximation of how much megabytes/sec you can get out
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of the drive.
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<p>(If you are a speed maniac and want a 10,000RPM drive for
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your cute little peecee, be my guest; however, those drives
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become extremely hot. Don't even think about it if you don't
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have a fan blowing air <em>directly at</em> the drive or a
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properly ventilated disk enclosure.)
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<p>Obviously, the latest 10,000RPM drives and 7,200RPM drives
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can deliver more data than the latest 5,400RPM drives, so if
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absolute bandwidth is the necessity for your applications, you
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have little choice but to get the faster drives. Also, if you
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need low latency, faster drives are better; not only do they
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usually have lower average seek times, but also the rotational
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delay is one place where slow-spinning drives can never beat a
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faster one. (The average rotational latency is half the time
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it takes to rotate the drive once; thus, it's 3 milliseconds
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for 10,000RPM drives, 4.2ms for 7,200RPM drives and 5.6ms for
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5,400RPM drives.) Latency is seek time plus rotational delay.
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Make sure you understand whether you need low latency or more
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accesses per second, though; in the latter case (e.g., news
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servers), it may not be optimal to purchase one big fast
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drive. You can achieve similar or even better results by
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using the ccd (concatenated disk) driver to create a striped
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disk array out of multiple slower drives for comparable
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overall cost.
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<p>Make sure you have adequate air flow around the drive,
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especially if you are going to use a fast-spinning drive. You
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generally need at least 1/2" (1.25cm) of spacing above and
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below a drive. Understand how the air flows through your PC
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case. Most cases have the power supply suck the air out of
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the back. See where the air flows in, and put the drive where
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it will have the largest volume of cool air flowing around it.
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You may need to seal some unwanted holes or add a new fan for
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effective cooling.
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<p>Another consideration is noise. Many 7,200 or faster drives
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generate a high-pitched whine which is quite unpleasant to
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most people. That, plus the extra fans often required for
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cooling, may make 7,200 or faster drives unsuitable for some
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office and home environments.
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<sect3><heading> Form factor</heading>
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<p>Most SCSI drives sold today are of 3.5" form factor. They
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come in two different heights; 1.6" ("half-height") or 1"
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("low-profile"). The half-height drive is the same height as a
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CD-ROM drive. However, don't forget the spacing rule
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mentioned in the previous section. If you have three standard
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3.5" drive bays, you will not be able to put three half-height
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drives in there (without frying them, that is).
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<sect3><heading> Interface</heading>
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<p>The majority of SCSI hard drives sold today are Ultra or
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Ultra-wide SCSI. The maximum bandwidth of Ultra SCSI is
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20MB/sec, and Ultra-wide SCSI is 40MB/sec. There is no
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difference in max cable length between Ultra and Ultra-wide;
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however, the more devices you have on the same bus, the sooner
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you will start having bus integrity problems. Unless you have
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a well-designed disk enclosure, it is not easy to make more
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than 5 or 6 Ultra SCSI drives work on a single bus.
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<p>On the other hand, if you need to connect many drives, going
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for Fast-wide SCSI may not be a bad idea. That will have the
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same max bandwidth as Ultra (narrow) SCSI, while
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electronically it's much easier to get it "right". My advice
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would be: if you want to connect many disks, get wide SCSI
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drives; they usually cost a little more but it may save you
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down the road. (Besides, if you can't afford the cost
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difference, you shouldn't be building a disk array.)
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<p>There are two variant of wide SCSI drives; 68-pin and 80-pin
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SCA (Single Connector Attach). The SCA drives don't have a
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separate 4-pin power connector, and also read the SCSI ID
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settings through the 80-pin connector. If you are really
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serious about building a large storage system, get SCA
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drives and a good SCA enclosure (dual power supply with at
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least one extra fan). They are more electronically sound than
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68-pin counterparts because there is no "stub" of the SCSI bus
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inside the disk canister as in arrays built from 68-pin
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drives. They are easier to install too (you just need to
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screw the drive in the canister, instead of trying to squeeze
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in your fingers in a tight place to hook up all the little
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cables (like the SCSI ID and disk activity LED lines).
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