doc/handbook/scsihd.sgml

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  <sect2><heading> SCSI hard drives</heading>
  <p><em>Contributed by &a.asami;.<newline>17 February 1998.</em></p>

      <p>As mentioned in the <ref id="scsi" name="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 <ref id="scsi" name="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.

    <sect3><heading> Rotational speed</heading>
      <p>Rotational speeds of SCSI drives sold today range from around
	4,500RPM to 10,000RPM. Most of them are either 5,400RPM or
	7,200RPM.  Even though the 7,200RPM drives can generally
	transfer data faster, they run considerably hotter than their
	5,400RPM 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 5,400RPM
	or slower drives.

      <p>Note that newer drives, with higher areal recording
	densities, can deliver much more bits per rotation than older
	ones.  Today's top-of-line 5,400RPM drives can sustain a
	throughput comparable to 7,200RPM 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'll get the
	rough approximation of how much megabytes/sec you can get out
	of the drive.

      <p>(If you are a speed maniac and want a 10,000RPM drive for
	your cute little peecee, be my guest; however, those drives
	become extremely hot.  Don't even think about it if you don't
	have a fan blowing air <em>directly at</em> the drive or a
	properly ventilated disk enclosure.)

      <p>Obviously, the latest 10,000RPM drives and 7,200RPM drives
	can deliver more data than the latest 5,400RPM 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's 3 milliseconds
	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.

      <p>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.

      <p>Another consideration is noise.  Many 7,200 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 7,200 or faster drives unsuitable for some
	office and home environments.

    <sect3><heading> Form factor</heading>
      <p>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
	CD-ROM drive.  However, don't 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).

    <sect3><heading> Interface</heading>
      <p>The majority of SCSI hard drives sold today are Ultra or
	Ultra-wide SCSI.  The maximum bandwidth of Ultra SCSI is
	20MB/sec, and Ultra-wide SCSI is 40MB/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.

      <p>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's much easier to get it "right".  My advice
	would be: if you want to connect many disks, get wide SCSI
	drives; they usually cost a little more but it may save you
	down the road.  (Besides, if you can't afford the cost
	difference, you shouldn't be building a disk array.)

      <p>There are two variant of wide SCSI drives; 68-pin and 80-pin
	SCA (Single Connector Attach).  The SCA drives don't 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).