Internet E-Mail Workshop/Internet E-Mail Workshop.iso/tutorial/pc_learn/hardrive.tut

Text File | 1993-08-16 | 28KB | 475 lines

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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 a 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.

       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). 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 protions 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.

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