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New section about SCSI hard drives. Not that I'm an expert or anything,

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but it was embarrassing to leave that entry empty!
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Satoshi Asami 1998-02-19 06:45:30 +00:00
parent 6142308768
commit 9342cdbb34
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=2453
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handbook/Makefile
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# $Id: Makefile,v 1.28 1997-11-18 16:06:04 wosch Exp $
# $Id: Makefile,v 1.29 1998-02-19 06:45:26 asami Exp $
SGMLOPTS=-links
SRCS= authors.sgml basics.sgml bibliography.sgml boothelp.sgml
@ -12,7 +12,8 @@ SRCS+= lists.sgml mail.sgml memoryuse.sgml
SRCS+= mirrors.sgml nfs.sgml nutshell.sgml pgpkeys.sgml policies.sgml
SRCS+= porting.sgml ports.sgml ppp.sgml printing.sgml
SRCS+= quotas.sgml relnotes.sgml routing.sgml russian.sgml
SRCS+= serial.sgml scsi.sgml sections.sgml sio.sgml skey.sgml slipc.sgml
SRCS+= serial.sgml scsi.sgml scsihd.sgml
SRCS+= sections.sgml sio.sgml skey.sgml slipc.sgml
SRCS+= slips.sgml stable.sgml submitters.sgml synching.sgml
SRCS+= term.sgml userppp.sgml uart.sgml linuxemu.sgml

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handbook/hw.sgml
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<!-- $Id: hw.sgml,v 1.78 1997-12-29 16:34:30 asami Exp $ -->
<!-- $Id: hw.sgml,v 1.79 1998-02-19 06:45:27 asami Exp $ -->
<!-- The FreeBSD Documentation Project -->
<!--
@ -451,7 +451,11 @@ Slippery when wet. Beware of dog.
<sect2><heading>* IDE</heading>
<sect2><heading>* Floppy</heading>
<sect1><heading>* Hard drives</heading>
<sect1><heading> Hard drives</heading>
&scsihd;
<sect2><heading>* IDE hard drives</heading>
<sect1><heading> Tape drives</heading>
<p><em>Contributed by &a.jmb;.<newline>2 July 1996.</em></p>
<sect2><heading> General tape access commands</heading>

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handbook/scsihd.sgml Normal file
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<!-- $Id -->
<!-- The FreeBSD Documentation Project -->
<!--
<!DOCTYPE chapt PUBLIC "-//FreeBSD//DTD linuxdoc//EN"> -->
<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).

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<!-- $Id: sections.sgml,v 1.26 1997-11-18 16:06:09 wosch Exp $ -->
<!-- $Id: sections.sgml,v 1.27 1998-02-19 06:45:30 asami Exp $ -->
<!-- The FreeBSD Documentation Project -->
<!-- Entities containing all the pieces of the handbook are -->
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<!ENTITY russian SYSTEM "russian.sgml">
<!ENTITY serial SYSTEM "serial.sgml">
<!ENTITY scsi SYSTEM "scsi.sgml">
<!ENTITY scsihd SYSTEM "scsihd.sgml">
<!ENTITY sio SYSTEM "sio.sgml">
<!ENTITY cy SYSTEM "cyclades.sgml">
<!ENTITY skey SYSTEM "skey.sgml">