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