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HARD DISKS - THE ESSENTIAL ACCESSORY <br />
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<br />
A simple observation: the first accessory any computer user <br />
should buy is hard drive. On a dollar for dollar basis nothing <br />
speeds up processing and expands convenience like a hard drive. <br />
The bad news? The substantial storage capacity of a hard drive <br />
contains the seeds of data catastrophe if you don't understand <br />
how to CAREFULLY maintain a hard drive. Some reference <br />
information pertaining to larger desktop hard drives as well as <br />
smaller laptop drives has been retained since drives in both <br />
computers are similar in function although different in form and <br />
size.<br />
<br />
Many computer operations tend to slow down at the critical <br />
bottleneck of information transfer from computer memory (RAM) to <br />
disk. The faster the transfer, the faster the program operates. <br />
Nine times out of ten it is the bottleneck formed when <br />
information flows to or from a disk that you and your program <br />
must wait. This is where a hard drive really shines - speed. <br />
<br />
Given the best possible treatment, a hard drive should last from <br />
eight to fifteen years. Drive manufacturers typically suggest <br />
30,000 to 70,000 hours of routine life for a hard drive before <br />
failure. If you kept your PC on for a 40 hour work week for 50 <br />
weeks - you could expect about 15 years of service for a drive <br />
rated at 30,000 hours. Some hard drive users even suggest <br />
leaving the drive on continuously or alternatively turning it on <br />
in the morning and off at night to minimize motor and bearing <br />
wear since it is the starting shock which wears most heavily on <br />
a drive. However, given marginal treatment or abuse, you can <br />
expect about fifteen minutes of service followed by a $250 <br />
repair bill. Obviously a little information about hard drives <br />
and their care can't hurt. <br />
<br />
---------------------------------------------------------------- <br />
<br />
TECHNOLOGY 101 - BOOT CAMP FOR HARD DRIVE USERS <br />
<br />
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<br />
What is a hard drive? If you have worked with a floppy disk you <br />
already understand something about hard drives. Basically the <br />
hard drive unit is a sealed chamber (sealed against dust and <br />
dirt) which contains rapidly spinning single or multiple stacked <br />
platters. The platter(s) are similar to a floppy disk in that <br />
they store information magnetically - data can be erased and <br />
rewritten as needed. The trick is, however, that the storage <br />
capability is immense on a hard drive. <br />
<br />
A floppy typically holds about one third of a million computer <br />
characters (360,000 or 360K bytes). The hard drive can commonly <br />
hold 20 to 40 million (or more!) bytes or computer words. In <br />
addition, the hard drive motor spins the magnetic platter <br />
quickly so that information is transferred rapidly rather than <br />
the tedious rate of the leisurely spinning floppy. A small <br />
read/write head hovers and moves above the hard drive magnetic <br />
platter much like a phonograph needle above a record. The <br />
difference is that the read/write head of the hard drive rides <br />
slightly above the platter on a thin cushion of air. In the <br />
floppy drive mechanism, the read/write head is in direct contact <br />
with the floppy. All hard drives are installed in two parts: the <br />
drive (a box containing the disk and read/write head) and the <br />
controller (a circuit board) which may be integrated into the <br />
drive or a separate circuit board. The hard drive stores the <br />
information. The controller assumes the role of a high speed <br />
"translator/traffic cop" to help the hard drive move its massive <br />
amount of information smoothly. <br />
<br />
Back to the magnetic platter for a moment. The read write heads <br />
are mounted on a moveable arm and each position of the head <br />
above the platter defines a circular TRACK just like the track <br />
of a phonograph record. As the arm changes positions, different <br />
circular tracks are traced magnetically upon the surface of the <br />
platter. Most hard drives have several read/write heads which <br />
service both the top and bottom of each platter. A set of tracks <br />
on different platters define a vertical CYLINDER somewhat like <br />
the surface of a tin can whose top and bottom are missing. Large <br />
hard drives can have six or more platters and therefore 12 or <br />
more sides for information storage. The tracks can also be <br />
defined as divisions of equally divided data called SECTORS <br />
which are something like portions of the outer edge of a circle. <br />
Finally, the sum collection of tracks, sectors and cylinders <br />
define the entire VOLUME of the hard disk. <br />
<br />
Each piece of data has an address which tells the read/write <br />
heads where to move to locate that specific piece of <br />
information. If you tell the read/write heads to move to and <br />
hover over a specific track, sooner or later your data will pass <br />
beneath it. Since you can move the heads directly to a given <br />
track quickly, the early nomenclature for a hard drive was the <br />
DASD or DIRECT ACCESS STORAGE DEVICE. <br />
<br />
Movement of the read/write head arm takes a little time. For <br />
this reason an ACCESS TIME is associated with hard drives and <br />
stated in advertising and specification sheets. Generally this <br />
time is stated as the AVERAGE ACCESS TIME and is frequently in <br />
the thousandths of seconds or millisecond range which is fast <br />
indeed. The old IBM XT class machines featured access times <br />
around 85 milliseconds with the AT class machines featuring <br />
access times around 40 seconds. Newer hard drives post times in <br />
the 28 to 15 millisecond access range. Remember, the faster you <br />
can move the read/write heads, the faster you can get to your <br />
data. <br />
<br />
The AVERAGE WAIT TIME is a less frequently discussed number but <br />
equally interesting. Once the read/write head is positioned over <br />
the track holding your data, the system must wait for the <br />
correct sector to pass beneath. Obviously, the average wait time <br />
is one half the time it takes for a full rotation of the <br />
platter. This figure is rarely given in advertisements and is <br />
usually comparable for most drives of the same type and is <br />
generally much shorter than the access time. Speed matters to a <br />
hard drive! Average wait time is published if you dig it out of <br />
the specification sheet or write to the manufacturer. <br />
<br />
An extension of this logic brings us to consider the INTERLEAVE <br />
FACTOR for a disk. Generally a hard drive reads and writes <br />
information in sectors of the same, repeatable size such as 512 <br />
bytes. However programs and data files are usually much bigger <br />
than this and obviously must be scattered onto many sectors. The <br />
problem is that the disk rotation is much too fast for a large <br />
file to be written in perfectly contiguous sectors on the same <br />
track. If you tried to write the data onto a track, one byte <br />
after the next, the central processing unit chip or CPU could <br />
not absorb the data fast enough. <br />
<br />
The solution is to place sectors to be read in ALTERNATING <br />
fashion which gives the CPU time to digest the data. Thus if a <br />
circular track on the platter had 8 sectors you might number and <br />
read them in this order: 1,5,2,6,3,7,4,8. This way the CPU has a <br />
"breather" in between each sector read. The number of rotations <br />
it takes the heads to read ALL tracks in succession is the <br />
INTERLEAVE FACTOR. Slow CPU chips can force a disk to use an <br />
interleave factor of 3 or even 4. A faster processor might be <br />
able to handle a disk interleave of 1:2 (such as 80286 processor <br />
chips) or even 1:1 (such as 80386 processor chips.) It is <br />
possible to low level format a disk and change its interleave <br />
factor; but if the CPU cannot keep up, the adjustment is <br />
worthless. To the processor operating in millionths of a second, <br />
the time drain of waiting for a hard drive which operates in <br />
thousandths of a second or floppy drive which operates in tenths <br />
and full seconds is wasted time. The obvious point of logic is <br />
that when using a hard drive you need to organize files for <br />
minimum time delays for the processor. <br />
<br />
The first outer track on a disk is always the boot record which <br />
loads the main portions of DOS into the machine. Following this <br />
is the file allocation table or FAT which we discussed in <br />
earlier tutorials. The FAT maintains data in CLUSTERS which, for <br />
an XT class machine are 4096 bytes. On the AT class machine the <br />
cluster size is 2048 bytes which is much more efficient and less <br />
wasteful of disk space. Following the FAT are the sectors for <br />
the root directory of the hard drive. Each directory entry is 32 <br />
bytes in length. Curiously, and to our good advantage, unused <br />
entries in the directory have a unique first character byte. <br />
When a file is deleted though DOS, ONLY the first character is <br />
reset. <br />
<br />
Fortunately this allows various utility programs to attempt to <br />
recover the deleted file since ONLY the directory data is <br />
altered but NOT the file itself. However, as time goes on and <br />
additional files are added to the disk, the original file is <br />
overwritten by new information. This is why you need to act <br />
immediately if you discover you have accidentally deleted a <br />
file. An advantage to the use of the FAT is that files do not <br />
have to be given a fixed amount of space on a disk - they can <br />
use as many or few clusters as needed. The downside is that the <br />
file pieces can be scattered wildly over the surface of the disk <br />
in a non contiguous fashion which only the FAT can track. This <br />
means more read/write head motion and more wasted time as far as <br />
the CPU and the performance of your program is concerned. <br />
<br />
Additionally, if you have many deleted files within the <br />
directory, DOS must search tediously through each one from top <br />
to bottom of the directory to find a match for the file you are <br />
trying to locate. Obviously, then, programs and data of high use <br />
should have their directory entries located near the top of the <br />
directory to speed the search. Each time the read/write head <br />
moves takes time: searching the directory and finding the pieces <br />
of the scattered file all take movement of the read/write arm. <br />
There are several ways to unfragment files which boost disk <br />
performance, and we'll talk about those techniques it a bit. <br />
<br />
---------------------------------------------------------------- <br />
<br />
HARD DISKS - STRATEGIES FOR TURBOCHARGED RESULTS <br />
<br />
---------------------------------------------------------------- <br />
<br />
Before we examine methods for improving hard drive performance, <br />
several simple "care and feeding" precautions should be <br />
mentioned. <br />
<br />
Hard drives are touchy if mistreated! Once brought up to speed, <br />
a hard drive should never be bumped or moved. The read/write <br />
head (similar to the phonograph needle resting on a record) will <br />
smash or chip into the surface of the spinning hard drive <br />
platter and take your data with it. Either the head or the <br />
magnetically coated platter can be permanently damaged. Allow <br />
the hard drive to some to a complete stop before moving the <br />
computer. <br />
<br />
In addition always use a "parking" software package to move the <br />
read/write head to the safety zone before turning off the <br />
computer. A parking program usually accompanies most computers <br />
which have hard drives installed or can be obtained from <br />
commercial or shareware sources. A few drives automatically park <br />
the heads when turned off but this tends to be a rare feature <br />
seen mostly on high priced hard drives. <br />
<br />
Always maintain copies of data and programs outside the hard <br />
drive by "backing up" onto a floppy or tape. How often should <br />
you back up your files? Daily if you use the computer to produce <br />
many changes to important documents. Weekly backup is probably a <br />
bare minimum considered reasonable for occasional computer <br />
users. Other computer users maintain vital data on floppies or <br />
other backup systems and use the hard drive to store programs or <br />
applications only such as a spreadsheet or database. Backups are <br />
a good idea even for floppy disk systems which have no hard <br />
drive. <br />
<br />
Make two copies of every file regardless of whether you have a <br />
hard drive or not. Some shareware and commercial utilities ease <br />
the backup chore by only copying those files to a floppy which <br />
have been changed or updated since the last backup has been <br />
performed. They ignore files which have not changed and thus do <br />
not require copying again. This can save a lot of time when <br />
backing up valuable files from your hard drive to a floppy for <br />
safekeeping. <br />
<br />
Hard drives should periodically be reorganized (files <br />
unfragmented) to ensure speedy retrieval and access to data. <br />
Inexpensive or free software programs known as "disk file <br />
unfragmenters" do this job nicely. As disk files are created and <br />
deleted, blank spaces and unused sectors begin to build up. <br />
<br />
Gradually files are broken into pieces and scattered over the <br />
many tracks and sectors of the disk. This happens to both <br />
floppies and hard drives, but is especially annoying on hard <br />
drives because of the dramatic increase in time it takes to load <br />
a program or data file. The File allocation table is the <br />
culprit, sense all data is packed away in the first and handiest <br />
sector on the drive which the FAT can find. <br />
<br />
The FAT allows files to be fragmented down to the cluster level. <br />
One way to unfragment a disk is to copy all of the files off to <br />
floppies and then recopy them back to the hard drive - a tedious <br />
nuisance at best. You would do this with the DOS XCOPY or COPY <br />
commands but not DISKCOPY since this would retain the tracks and <br />
their fragmentation as you first found them. <br />
<br />
Defragmenting programs perform this task without requiring <br />
removal of the files from the hard drive. They perform their <br />
magic by moving around the clusters of a scattered file in such <br />
a way as to reassemble it into contiguous pieces again. Some <br />
customization is permitted with the more sophisticated <br />
"defragmenting" programs. For example, subdirectory files can be <br />
relocated after the root or below a different subdirectory or, <br />
in another example, high use files might be placed higher in the <br />
directory listing for faster disk access. <br />
<br />
The first time a defragmenting program is run may require <br />
several hours if a hard drive is large and badly fractured with <br />
scattered files and clusters. It is a good idea to backup all <br />
essential files prior to "defragging" just in case there is a <br />
power failure during a long "defrag". Subsequent runs of the <br />
"defragger" produce runs of only a few minutes or so since the <br />
heavy work was done earlier. Essentially, "defragging" the hard <br />
drive should be done regularaly, perhaps weekly. Defragging is <br />
not a substitute for caching, ramdisks, or buffer - instead it <br />
is a maintenance function which should be done regularly. <br />
<br />
Yet another possible avenue to improve disk performance is that <br />
of changing the disk interleave factor which we will discuss a <br />
bit later in this tutorial. By way of brief introduction: the <br />
disk interleave indicates how many revolutions of the magnetic <br />
platter are required to read all the sectors of data from the <br />
spinning track. A ratio of 1:1 means all data can be read <br />
sequentially. One sector of data after another. <br />
<br />
There is some overhead time required for the read/write head to <br />
zip to the FAT area of the disk (if it is not in a cache or <br />
buffer) to determine location of the next sector along the disk <br />
track. <br />
<br />
For example, five clusters of data on a track might require four <br />
trips back to the FAT track to find the cluster addresses even <br />
on a completely defragmented disk. We will talk more about <br />
cluster and defragmenting a bit later in this tutorial. <br />
<br />
Nevertheless, depending on the speed of your central processor <br />
or CPU, using a program which tests and alters the interleave <br />
factor, IF THIS CAN BE DONE, may yield better performance. Most <br />
interleave adjustment software first performs a test to <br />
determine the current interleave, the possible changes and of <br />
course how much performance time might be gained. A few of these <br />
packages can alter the interleave with the files in place but <br />
you should backup truly essential files before starting the <br />
process. Interleave factor adjustment are mainly derived from <br />
the CPU speed NOT the disk speed. Thus a fast AT or 80386 <br />
equipped machine will more likely be able to take advantage of <br />
an interleave adjustment. <br />
<br />
Tinkering with a hard drive for optimum results might best be <br />
divided into two categories: DISK SUBSTITUTION and DISK <br />
ALTERATION. DOS allows two clever ways substituting RAM memory <br />
for disk memory. <br />
<br />
In the first, using BUFFERS, the small CONFIG.SYS file on your <br />
hard drive is modified to contain a buffers statement. A sample <br />
might be: BUFFERS=20. A DOS buffer is an area of RAM memory <br />
capable of holding a 512 byte mirror image of a disk sector. <br />
This allows DOS to quickly search the buffer area for frequently <br />
used data instead of the slower disk. In the older XT class <br />
machine, if you did not specify a buffer size, DOS defaulted to <br />
2 buffers while later versions of DOS default to about 10 <br />
buffers. Most users settle on about 20 buffers but you can <br />
specify up to 99 with current releases of DOS. But you don't get <br />
something for nothing. If you used the full 99 buffers <br />
available, you would soak up 45K of your main RAM memory! The <br />
downside of using buffers is that more is not necessarily <br />
better. <br />
<br />
Unfortunately, DOS searches the buffer area of RAM sequentially <br />
rather than logically so if DOS requires data which is in the <br />
buffer area, it will search each 512 byte area in sequence from <br />
top to bottom even though the data it needs may be at the end of <br />
the buffer. Logically, then, there is an optimum number of <br />
buffers - too many used with a small program and you can slow <br />
things down, not enough and DOS will be forced to go out to the <br />
disk to retrieve what it needs. If you rarely use the same data <br />
within a program twice but load lots of different programs and <br />
data, a large number of buffers won't help. However if you need <br />
frequent access to a certain data file or portion of that file, <br />
buffers will help. Portions of the FAT are kept within the <br />
buffers area, so dropping your buffers to zero has the damaging <br />
effect that DOS must always go to the disk to read the FAT which <br />
isn't helpful either. <br />
<br />
Another way of substituting RAM memory for disk memory involves <br />
using a RAMDISK. The idea is to create in RAM memory an entire <br />
disk or a small portion of a disk. This works like magic on many <br />
machines since the reading of tracks and sectors takes place at <br />
the high speed of RAM memory rather than the mechanically <br />
limited speed of the read/write heads on a floppy or hard drive. <br />
<br />
But be careful. Three areas of difficulty can arise. First you <br />
must remember to take the data from a floppy or hard drive and <br />
move it into the RAMDISK. Many people do this automatically from <br />
within an AUTOEXEC.BAT file or may have several floppies, each <br />
with a different RAMDISK configuration depending on the task at <br />
hand. Copying data to the RAMDISK usually moves along briskly. <br />
Secondly you must sacrifice a large area of memory for the <br />
RAMDISK which can no longer be used by your main program. Users <br />
of computers with extended or expanded memory usually choose to <br />
put their RAMDISK in the extended or expanded memory area of RAM <br />
so that precious main memory is not lost. Still, a small RAMDISK <br />
can soak up 64K of RAM memory and one or two MEG RAMDISKS area <br />
common for many users. The third and most serious problem when <br />
using RAMDISKS is that they are volatile - switch off the <br />
machine or experience a power failure, and your data is lost <br />
forever! Rather than residing safely on a magnetic disk, the <br />
data is "floating" in RAM memory and should be - MUST BE! - <br />
written to a disk before the machine is powered down. <br />
<br />
Many applications fly with a RAMDISK. Users of word processors <br />
find that moving the spelling checker and thesaurus to the <br />
RAMDISK speeds up things considerably since these are used <br />
heavily in a random manner. Spreadsheet users find that reading <br />
and writing short data files to RAMDISKS is a boon. Programs <br />
which use overlay files or temporary files as well as <br />
programming compilers benefit from RAMDISK use. Batch files <br />
which are disk intensive as well as small utilities really <br />
sprint when placed on a RAMDISK. Basically, any program file <br />
which is frequently used and loaded/unloaded repeatedly to a <br />
disk during normal computer operation is an excellent candidate <br />
for RAMDISK placement. DOS contains a RAMDISK which is called by <br />
using the statement DEVICE=VDISK.SYS or DEVICE=RAMDRIVE.SYS (if <br />
you are using MSDOS) which is placed in your CONFIG.SYS file. <br />
Your DOS manual details the specifics such as stating the size <br />
of RAMDISK and giving it a drive letter. You must still copy <br />
your target files into the RAMDISK and place it in the search <br />
path (with the PATH= command) as we mentioned in a previous <br />
tutorial. And the RAMDISK should always be the first drive <br />
letter mentioned in the path command so that DOS searches it <br />
first for optimum results. <br />
<br />
Yet another area of investigation is that of CACHE software. <br />
Essentially a CACHE is an extension of the buffers idea we <br />
discussed earlier. But the twist is that the CACHE is searched <br />
intelligently by a searching algorithm within the CACHE software <br />
rather than from top to bottom as with the more typical DOS <br />
buffer search system. Disk CACHE software can be obtained as <br />
either commercial software or shareware. As with a RAMDISK, the <br />
CACHE requires a chunk of RAM memory to operate. This can be <br />
extended memory, expanded memory or main RAM memory. Some <br />
manufacturers include a CACHE program with the software package <br />
or DOS disk. A CACHE is a sophisticated type of RAMDISK, in a <br />
rough sense. <br />
<br />
CACHE software allocates a large area of memory for storage of <br />
frequently used disk data. This data is updated by an <br />
intelligent CACHE search algorithm in an attempt to "guess" <br />
which tracks of a disk you might read or need next. The CACHE <br />
also stores the most frequently used disk data and attempts to <br />
remove less frequently used data. Whenever DOS requests disk <br />
data, the CACHE software first tries to fill the order from data <br />
currently stashed in the CACHE which prevents a slower disk <br />
search. <br />
<br />
When data is written from the program to the CACHE, first a disk <br />
write is done to prevent data loss in case of power failure and <br />
then the data is stashed in the CACHE in case it is needed <br />
again. Usually the hard drive data is the target of the CACHE <br />
activity, but a floppy disk could also be cached. All CACHE <br />
software allows you to allocate the size of the CACHE as well as <br />
the drive or drives to be cached. And some even allow you to <br />
specify exact files or data to be cached. The key is that high <br />
use data lives in RAM memory which keeps tedious disk access <br />
times low. In general, if your computer has a megabyte or more <br />
of memory and a speedy processor such as an 80286 or 80386 <br />
either or both a CACHE or RAMDISK option does improve <br />
performance. <br />
<br />
As we leave hard disk boot camp, let's finally look at hard <br />
drive formatting processes. Two basic formatting operations are <br />
of concern: physical formatting or low level formatting and <br />
logical or high level formatting. When you use the format <br />
program on a floppy disk both low level and high level <br />
formatting is accomplished. On a hard disk, formatting performs <br />
only logical or high level formatting. On a hard disk, low level <br />
formatting is usually done to a disk before shipment. As an <br />
aside, the FDISK command of DOS has little to do with either <br />
type of formatting, but is a method of partitioning or arranging <br />
the data onto the hard drive tracks. Each disk platter is <br />
separated into circular concentric tracks where data is stored <br />
as we saw earlier. During physical formatting the tracks are <br />
divided into further subdivisions called clusters and further <br />
yet into sectors. High level formatting involves the specific <br />
ordering of the space for the exclusive use of DOS and is a bit <br />
more analogous to the formatting of a floppy disk. <br />
<br />
Some software programs of use by hard drive owners: <br />
<br />
The following two programs perform low level formatting and <br />
simple diagnostic routines on a hard drive: <br />
<br />
Disk Manager and CheckIt <br />
<br />
Data recovery and "unerasing" programs also containing <br />
diagnostic routines are: <br />
<br />
PC Tools Deluxe, Norton Utilities, Mace Utilities <br />
<br />
Extensive diagnostic and maintenance/data repair functions as <br />
well as interleave alteration and head parking are offered by: <br />
<br />
SpinRite II, Optune, Disk Technician <br />
<br />
Shareware programs with unerase functions include: <br />
<br />
Bakers Dozen <br />
<br />
Shareware programs with defragmentation capabilities include: <br />
<br />
SST and PACKDISK. <br />
<br />
Tutorial finished. Be sure to order your FOUR BONUS DISKS which <br />
expand this software package with vital tools, updates and <br />
additional tutorial material for laptop users! Send $20.00 to <br />
Seattle Scientific Photography, Department LAP, PO Box 1506, <br />
Mercer Island, WA 98040. Bonus disks shipped promptly! Some <br />
portions of this software package use sections from the larger <br />
PC-Learn tutorial system which you will also receive with your <br />
order. Modifications, custom program versions, site and LAN <br />
licenses of this package for business or corporate use are <br />
possible, contact the author. This software is shareware - an <br />
honor system which means TRY BEFORE YOU BUY. Press escape key to <br />
return to menu. <br />
</pre><br />
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[[Category:Computing]]</div>Netfreak