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  1. —————————————————————

HARD DISKS - THE ESSENTIAL ACCESSORY

  1. —————————————————————
     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. 
  1. —————————————————————

TECHNOLOGY 101 - BOOT CAMP FOR HARD DRIVE USERS

  1. —————————————————————
     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. 
  1. —————————————————————

HARD DISKS - STRATEGIES FOR TURBOCHARGED RESULTS

  1. —————————————————————
     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 
     return to menu. 
     
/data/webs/external/dokuwiki/data/pages/archive/computers/hd.txt · Last modified: 2001/08/19 02:43 by 127.0.0.1

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