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**************************************************************** * * * THE GUIDE TO HARD DISK SECTOR INTERLEAVING * * * * by * * Steven Gibson * * GIBSON RESEARCH CORP. * * Box 6024, Dept C * * Irvine, CA, 92716 * * (714) 854-1520 * * * **************************************************************** In these "sophisticated" days of computers, where AUTOEXEC.BAT and CONFIG.SYS make casual conversation, it's rare to find a topic as interesting and critical but still mis-understood, as hard disk sector interleaving. Our researches into this area have discovered that MOST IBM AND COMPATIBLE personal computers are performance-crippled by mis- interleaved hard disk drives. Unfortunately, many disk controller companies, in competing with each other, have set their disk interleave defaults too tightly for many computers. Such "specsmanship" directly hurts the innocent computer user (you) by dramatically limiting his hard disk data transfer rate. This GUIDE carefully explains the situation and shows how to use the two included programs to determine whether your own IBM PC or compatible's hard disk drives have their sector interleave set correctly. Responding to this problem, Gibson Research Corp., publisher of the popular display screen enhancement utility FlickerFree, has recently developed an inexpensive software SOLUTION which first determines your system's optimum hard disk interleave factor then RESETS IT while leaving all your hard disk data in place! UNDERSTANDING HARD DISK SECTOR INTERLEAVING It's a rare person who would not wish for additional performance from his personal computer's hard disk drive. While much attention is given to the drive's Average Seek Time, which is a measure of the time required to move the read/write head from one track to another, there is another vital detail which determines overall hard disk performance and which is subject to the user's control. We will see that the too often neglected SECTOR INTERLEAVING factor of a hard disk has a dramatic impact on data transfer rates. As most people know, the information stored on a floppy or hard disk is arranged in a series of concentric circular paths called tracks. The disk drive's read/write head is specifically positioned over any desired track with an operation called a SEEK. Thus an obvious limit on the speed with which a drive can find or place information would be the so called track-to-track and average seek times. A single track of a standard IBM compatible PC contains approximately nine thousand bytes of data. But since we usually deal with data in much smaller chunks, each track is divided into smaller sections called sectors. Think of a spinning pizza which has been cut into seventeen identical, and numbered, slices. (Drives with RLL encoding pack 50% more data onto every track resulting in more than thirteen thousand bytes per track divided into 25 or 26 sectors.) Now suppose that we need to read the information contained in sector 1 of our current track. We patiently wait for sector 1 to rotate under our read/write head, reading its data at that time. After absorbing this freshly read information, we realize that we also need to read the next sector, number 2. However, by the time sector 1 has been moved into our computer and we've decided to read sector 2, the beginning of sector 2 has already started passing under the read/write head. So we have no choice but to wait for the disk to rotate all the way around once more to deliver sector 2. If we wished to read a nine thousand byte file composed of all seventeen disk sectors on this track, seventeen complete rotations of the disk, one for each sector, would be required! It wasn't long before a bright engineer realized that the entire problem could be easily resolved by spreading the sequentially numbered sectors out around the disk: Instead of placing sector 2 immediately after sector 1, sector 2 could be placed a few sectors later! In this way, after reading sector 1, sector 2 would be just about ready for reading by the time we were ready for it. Such an elegant solution! If, for example, sequentially (logically) numbered sectors were staggered out every three physical sectors, then each rotation of the disk could read every third sector. Therefore only three revolutions of the disk would be required to read an entire track. Quite an improvement over 17 revolutions! This sector staggering is known as SECTOR INTERLEAVING or SECTOR MAPPING. The physical spacing between logically consecutive sectors is known as the INTERLEAVE FACTOR. This example used an interleave factor of three, shown as "3:1" and pronounced: "3 to 1". The new higher-density RLL controllers need to be correctly interleaved too. With 26 sectors per track a non-interleaved or mis-interleaved disk would require 26 revolutions for an entire track transfer! Now here's the real rub: In the current environment of mix and match highly modular personal computing, responsibility for determining and setting your hard disk drive's optimal sector interleave factor has "fallen through the cracks" as it were. You've never worried about it have you? If you're inclined to believe that someone else has, (like your local dealer perhaps) you might be in for a real surprise. Experiments with a wide variety of computers, drives, controllers, clock speeds, and interleave factors have shown that the hard disks of MOST PERSONAL COMPUTER SYSTEMS ARE NOT PROPERLY INTERLEAVED TODAY! So many computers are so badly interleaved that it's quite likely that you could increase your own hard disk's performance, by FOUR TO SEVEN TIMES just by optimally setting your disk's interleave factor! The interleave factor can be either too "loose" or too "tight". The result of operating with an interleave factor which is too loose is lower performance than a particular drive-controller- computer combination could achieve with tighter interleaving. For example the original IBM PC/XT is interleaved at 6:1 but can readily achieve 5:1 in a standard 4.77 Mhz machine and 4:1 in an 8 Mhz PC. This means that disks on accelerated PC's can read and write at 150% of their current rate! The consequence of operating with an interleave factor which is too tight is more disastrous, since missing that next sector induces the significant delay of another entire disk revolution! You might be saying to yourself "so what's an extra disk revolution between friends?", but consider this: If your disk is properly interleaved, say with a factor of 3, the entire nine thousand byte track will be read or written in just 3 disk revolutions. However, if the particular controller in your particular computer cannot achieve that interleave of 3, then 17 or 18 entire revolutions will be required to read the same track. 3 revolutions versus 18 is a performance difference of 600%! Since many files and most programs are well over nine thousand bytes long, and since DOS DOES REQUEST an entire track transfer in such cases, these 17-revolution track reads add up quickly. You would certainly feel the difference between waiting 3 seconds for a program to start up, versus 18 seconds! The damning thing about proper hard disk sector interleave is its total dependency upon EVERY factor of your system. For example, the AT&T 6300 requires 20 REVOLUTIONS for a full track transfer when reading a disk which is interleaved at the most common factor of 3:1! Even it's faster brother, the 6300 Plus, requires 14 revs! We have found that the AT&T's hard disk data transfer rate can be made FOUR TIMES FASTER when its interleave is optimized for it! If you're using a 6300 or 6300+, your disk transfer rate is one quarter of what it could be! Another example: Western Digital's RLL hard disk controller formats drives at a default interleave of 3. Believe it or not, this interleave is too tight for good old standard 4.77 Mhz XT! Consequently 28 entire disk revolutions are required to read a SINGLE track! If the interleave is set to 4 then only 4 revs. are required to read the same data, for a 700% throughput boost! So by now you're dying to know what's happening with the hard disk sector interleave of your own computers! What interleave factors are being used, are they correct, and what you can do about it if not! Included with this INTLEAVE.DOC file are two small programs: SPINTEST.COM and SPINTIME.COM The main program, SPINTEST.COM determines how many disk revolutions your IBM compatible personal computer requires to read an entire 17 sector track from its hard disk. (SPINTEST is also fully compatible with the 25 or 26 sector tracks used with the high-density RLL controllers.) Running SPINTEST on the many computers at Gibson Research, and in many local computer dealerships, we discovered something quite surprising: Most computers being sold today are horribly interleaved. We turned up many machines which required more than 17 entire revolutions just to read or write one full track! These same machines flew along at 4:1 but not at 3:1. For example, Gibson Research has a vintage Leading Edge Model-D with a standard Western Digital hard disk controller. This machine always seemed to run more quickly than a newer generic PC clone equipped with a different Western Digital controller. Both hard disks received their original low-level (interleaving) format at the Western Digital default interleave of 3. After all, those dealers must have thought, Western Digital knows what's best. Right? NOT NECESSARILY! Since the controller AND computer together determine the optimal interleave, it is IMPOSSIBLE for any controller manufacturer to set a single optimal interleave for everyone. In fact, even the controller's revision level can be a determiner! One of our two WD controllers ALWAYS MISSES sectors when the disk is inter- leaved at 3, while the other controller makes 3:1 ... in the same computer! To test this, one of our hard disks was given a low-level format at an interleave of four. Afterward, the SPINTEST timing diagnostic program and a stop watch confirmed our suspicions: An MS-DOS file copy operation on the hard disk which had required 139 seconds when the disk was interleaved at 3:1 now took only 39 seconds at an interleave of 4:1! Interestingly, that "slower" controller which can't make the interleave of 3 on a standard 4.77 Mhz XT does just fine on a faster 8 Mhz clone computer. So as you can see, the COMPUTER makes a difference too! Any way you look at it, the issue of sector interleaving involves the complex interactions among the disk drive, the controller make and model (even the revision level), the processor, and system clock rate. OVERVIEW OF THE INTERLEAVE DIAGNOSTIC PROGRAMS The issues of hard disk sector interleaving have gone unnoticed for so long because there has never been a clear way to see what's really going on deep inside a hard disk. After seeing the importance of this issue, we decided to change this. The two programs SPINTEST and SPINTIME determine, for any standard MFM or RLL controller, exactly how many revolutions the disk requires to transfer one entire track of data. The programs were hand written in 100% machine language (as is everything Steve Gibson writes and Gibson Research publishes) to give them the required measuring resolution speed. THE SPINTEST PROGRAM SPINTEST determines exactly how many revolutions your hard disk currently requires to perform a full track transfer. DOS transfers a full track whenever programs or files larger than 9K are read or written ... which is most of the time. SPINTEST does NOT directly read your drive's interleave, but the drives interleaving performance can be easily inferred from SPINTEST's full track transfer revolution count. SPINTEST averages the time required for each of over two hundred full track reads in order to accurately determine the revolution count per read, then the number of revolutions required to read just one track and the controller's full-track data transfer rate are computed. SPINTEST only reads data from your drive so data is NEVER altered. Over two hundred track reads were used because of the inconsistent disk transfer behavior of AT&T's 6300 machines. These exhibit a maddening inconsistency in their ability to transfer disk data. Sometimes they get the next sector, and often not. This means that a much looser than normal interleave generates MUCH BETTER overall disk throughput for AT&Ts. For this reason, and to guarantee correct results on any machines which may behave similarly, SPINTEST performs many track reads and averages the results. THE SPINTIME PROGRAM Since SPINTEST measures track read time, it must assume a given ratio between elapsed time and disk rotation rate. Some weirdo clone computers have a clock which runs faster than normal when in their "turbo" (8 Mhz) mode, and some lap computers have tiny hard disks spinning at weird speeds. If these do not sound like your situation SPINTEST will deliver correct readings and you'll not need SPINTIME's confirming measurements. But if EITHER of these cases might be you (if you have a weirdo clone computer or hard disk sporting laptop) SPINTIME will tell you for sure. SPINTEST is only usable when SPINTIME gives standard readings. The single (not surprising) exception to this is for the AT&T 6300 and 6300 Plus machines. Due the overall problems they experience with disk transfers, SPINTIME may show a lower-than-3600 RPM reading. For 6300's this is normal. SPINTIME is interesting regardless since it determines EXACTLY how fast your hard disk is really spinning! SPINTEST To run SPINTEST, simply type its name at the DOS prompt. IMPORTANT!: Read the section IMPORTANT SPINTEST AND SPINTIME NOTES below, before you begin! Since SPINTEST takes the average of over 200 track reads, the time required to run this test will vary between approximately 14 seconds for a fast 2:1 interleaved machine to 98 seconds for an incorrectly interleaved 26-sector RLL encoded hard disk. So simply type: SPINTEST at the DOS prompt and wait a minute or two. SPINTIME SPINTIME has two purposes: To determine the speed of your system's clock (if you suspect that it might be weird) , and to verify that your system's hard disk is spinning at close to the standard 3600 RPM standard. Your system's clock is checked by measuring SPINTIME's exact execution time. It should require EXACTLY 60 SECONDS to run ... no matter what. During that time, it is busy watching your hard disk spin, counting every revolu- tion. After EXACTLY 60 seconds, it displays your drive's exact RPM (within its measuring resolution.) IF SPINTIME's total execution time is NOT EXACTLY 60 seconds, OR the displayed RPM is not close to 3600, SPINTEST's reported revolution count won't mean anything either. SPINTIME's real value is to assure you that SPINTEST is delivering worthwhile answers. IMPORTANT SPINTEST AND SPINTIME NOTES! Both programs assume and require DOS 2.0 or higher. They operate on your system's FIRST hard disk only (drive C). The hard disk must be bootable, have its controller plugged into a slot, not requiring special CONFIG.SYS device drivers for operation. SPINTEST and SPINTIME will never disturb your hard disk data, but if your disk is some kind of odd-ball they may not function correctly. The tests will be more accurate if your normal collection of memory resident programs (if any) are not in memory at the time. Many resident programs "steal" so much time from the computer that a delicate timing analysis could be upset. It would be wise to boot your machine from a plain DOS diskette when running these tests for maximum reliability. Also, turn off (or better yet remove) any disk caching software you might be using. Disk caching intercepts the disk reading performed by the programs and renders their measurements invalid (and rather humorous). By the way, disk caching program per- formance is significantly improved through proper disk interleaving. WHAT IT ALL MEANS So now you're wondering: What do the numbers mean? How do they compare with industry norms and everyone else's? Should I be dancing on roof tops or wringing my dealer's neck? A hard disk spins at 3600 revolutions per minute, or 60 revolutions per second. A track has 17 sectors of 512 bytes per sector. This means that data passes under your drive's head at a rate of 522,240 bytes per second. (This number is 768,000 for RLL con-trollers with 25-sector tracks.) Your system will achieve some fraction of this maximum possible rate as determined by the number of rotations required to read or write each track. 522,240 (or 768,000) is divided by the number of revs per track (as SPINTEST does) to calculate your drive's data transfer rate. Since your system's performance is meaningful only when compared to other properly and improperly interleaved systems, the following table will give you a feeling for where the industry stands: 1:1 - There IS a controller which achieves 1:1, unfortunately it lacks Error Correcting Code (ECC) which is critical for reliable operation. 2:1 - Compac machines achieve 2:1. Adaptec's 2070A RLL controller can do 2:1 with an 8 Mhz machine even though their low-level formatter stubbornly refuses to allow a setting of less than 3! 3:1 - Newer WD controllers and many other controllers will handle 3:1. The Adaptec 2070A RLL con- troller will do 3:1 in a 4.77 Mhz machine. 4:1 - Older WD controllers need 4:1, original IBM XT controllers can do 4:1 in an 8 Mhz machine. The AT&T 6300 Plus works best at 4:1 and WD's RLL controller can only do 4:1. 5:1 - IBM's original XT controller can do 5:1 in a 4.77 Mhz XT but comes interleaved at 6:1. The AT&T 6300 is best at 5:1 which increases over- all throughput 400% over its default! 6:1 - Only the original IBM PC/XT seems to have been way out here. These machines can be tightened up, and if it has an inexpensive accelerator it can probably run at 4:1 for 150% disk boost. We have seen that WD controllers which miss at 3:1 will do beautifully at 4:1. Several InfoWorld readers have independently confirmed that AT&T machines (with WD controllers) perform best at the very loose interleave of 6:1. Even the old true blue IBM controller which runs at 6:1 can always do 5:1, and even 4:1 in a faster than 4.77 Mhz machine. DETERMINING FACTORS So what system factors influence and determine the optimal interleave setting for a given set of equipment? After a sector of data has been read it must be moved from the controller's on- board buffer into the computer. This is done with a process known as DMA (Direct Memory Access.) The time required to transfer the sector determines how soon the controller will be ready to read the next sector. For this reason add-in Accelerator "Turbo" Cards do not generally change a machine's optimum interleave since the main system clock speed, which continues to control DMA memory accesses, is not changed. The less expensive "Speed Booster" products which alter the basic system clock timing DO have a tremendous impact on optimal interleave by running the DMA faster and thus moving the data in and out faster. GIBSON RESEARCH AND INTERLEAVING When we began these experiments we had NO IDEA that so many personal computers were so poorly interleaved. Without the aid of our new special software, changing a hard disk's sector interleave "manually" requires first backing-up all hard disk data onto some secure medium. Then a low-level re-formatting is performed. This unfortunately messy task was never designed for the casual computer user since it requires unpublished knowledge of the internal details of your controller, using the DOS DEBUG command to poke hexadecimal values into the 8088's machine registers and starting the low-level format. After this, the FDISK and FORMAT commands are used to create a partition table and lay down the high-level formatting information. After all this, the backed up data must be copied back onto the drive. Then, if the experimentally chosen interleave was not correct, most of the process would have to be repeated with a different trial interleave factor. Gibson Research Corp., responding to the clear need for a better solution to the task of hard disk sector interleaving optimiza- tion has automated and streamlined this "re-interleaving" process with a product which was designed specifically to meet this need. This software product quickly determines the optimal interleave setting for any disk of any size in any system with any clock rate. Once determined, the existing interleave of the disk can be RESET automatically, and in just a few minutes, leaving all your disk data intact and in place! The product also performs several other useful hard disk utility functions which have never before been available. GET YOUR DISK SPINNING RIGHT!! Even if SPINTEST turned in excellent results of 3 or 4 revs, you should STILL check out Gibson Research's new product for the other surprising (NEVER before offered) hard disk capabilities it brings ... and if you received results like 10, 12, or even 17, 18, or 19 revolutions, there is NO DOUBT that your system could be running FOUR TO FIVE TIMES FASTER ... in minutes! To receive all the details about this product simply phone, or mail your name and address. Ask for the INTERLEAVING INFO from the On-Line edition of the GUIDE. ... you'll be glad you did!! GIBSON RESEARCH CORP. Box 6024, Dept C Irvine, CA 92716 (714) 854-1520 (If you're wondering about Gibson Research's first product, FlickerFree, ask for some info about it too!) **************************************************************** * * * <-- The End ... Of the GUIDE --> * * * ****************************************************************