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Text File | 1989-12-28 | 12.9 KB | 224 lines

TESTDISK THE NEXT GENERATION OF DISK THROUGHPUT BENCHMARKS (c) COPYRIGHT 1988-1989 BY COLUMBIA DATA PRODUCTS, INC. 1070B RAINER DRIVE P.O. BOX 2584 ALTAMONTE SPRINGS, FL 32714-2584 (407) 869-6700 Welcome to the world of hard disk performance measurement! Until recently, measuring performance of disk subsystems for the IBM PC/XT/AT/PS2 and compatibles has been somewhat over simplified by the industry. Now, with the introduction of SST-equipped controllers, contemporary benchmarks are no longer adequate to predict disk subsystem performance in actual everyday use. Benchmarks for the next generation of mass storage systems for the IBM PC/XT/AT/PS2 and compatibles now require that more variables be taken into account to accurately measure the throughput a system can achieve. ACCESS TIMES Until now the only major variable that generally was considered was the Average Disk Head Seek Time, expressed in milliseconds (commonly referred to as Average Access Time). From the standpoint of a disk drive manufacturer, this is the major item which is used to market their products and they are consistently touting faster and faster access times. But the REALLY important aspects of overall disk system performance are just not shown. Let's take a disk that has an access time of 20 milliseconds, for example. Does that mean that just because it has a 20 millisecond access time that it is fast? Well, yes and no. It means that it can move the disk head from one spot on the disk to another spot on the disk in an overall average of 20 milliseconds. As far as access times go, a 20 millisecond average access time disk drive is relatively fast. But it tells you nothing of how fast it will deliver data to your computer. So you ask, "Well, then, how can I tell when a disk drive is fast?" Read on!! Disk performance really starts at your hard disk controller--not at your disk. Disk performance is a function of how rapidly data can be transferred from the hard disk into your computer. If you have a disk controller which can move data at 29,000 characters per second (which is not uncommon), then it stands to reason that it will take you six seconds to load your 174,000 character spreadsheet. So, whether you have a 20 millisecond hard disk or a 65 millisecond hard disk, it still takes six seconds to recover your spreadsheet!! DATA RATES As you can probably tell, disk performance measurement, which tells you how long you are going to wait for something to load into your computer from your hard disk, can best be expressed in characters per second. Since one character is equal to one byte, and most disk systems move data in thousands of bytes (or characters) per second, the standard term for data rate is kilobytes (1024 bytes) per second (KBS). Some benchmark tests actually DO report the data transfer rate (in KBS) of the controller. They measure the movement of data from the disk to memory. They only report, however, the highest data rate your disk drive can achieve on your computer. Now, if you read closely, you will notice that I said the HIGHEST data rate your disk drive can achieve on your computer. Does this mean that there is more than one data rate expressed in KBS which can be achieved by your disk system? The answer is YES! WHICH DATA RATES? Whenever the disk system is asked to move data from the hard drive into memory or vice versa, it takes time for the disk system to figure out what you are asking for, where it is on the disk, and how to move the data. This time lag is called command processing latency. It is this initial time lag before data is actually moved to or from the disk that is the evil of disk performance. Since this time lag is pretty much a constant, it is wise to get as much data off of the disk at one time rather then to repeatedly ask for data in small blocks. For example, if you wanted to move 327,680 bytes off the disk and the initial command processing latency was 70 milliseconds (70/1000 of a second) and the 327,680 bytes was moved in 512 byte blocks, you would be asking the disk to move data 640 times and your command processing latency alone would be 44.8 seconds (640 X .070 seconds)!! If, however, you moved the 327,680 bytes off the disk 65,536 bytes (64 KB) at a time, you would be asking the disk to move data only 5 times and your command processing latency would be 35/100's of a second (5 X .070 seconds). That is 992% faster!!! 44.8 seconds compared to a fraction of a second and that's not even counting the time to move the data!! This is extremely eye opening isn't it? So, you see, it doesn't appear to be very wise to ask the disk for data in small blocks, now does it? But this is precisely the way most application programs (such as Lotus, Dbase, etc) DO move their data--512 bytes at a time. Now, these other disk performance testers I mentioned, test the disk by moving data 65,536 bytes at a time. So, you have absolutely no idea how fast Lotus, Dbase, or any of the other application programs can retrieve data. All you know, is that IF YOUR PROGRAM MOVED DATA 64K AT A TIME OFF THE DISK, THIS IS HOW FAST IT WOULD BE!! In essence, then, to measure disk performance in 64K blocks is utterly useless to you if you really want to know how fast your application program can load data. Your Columbia Data Products disk performance tester, TESTDISK, doesn't just move data off the disk in 64K blocks. It moves the data in 8 different size blocks: 1/2K, 1K, 2K, 4K, 8k, 16K, 32K and 64K. This will give you a clear idea of how fast your disk system really is. When you run this on a disk system which is 650 KBS at 64K blocks, you may be surprised to find that it is only 120 KBS (or less) at the 1/2K block rate. You may even find that you paid a premium for a "fast" disk system and you never will see the "speed" because the programs you run cannot utilize it. One last word about average access times. They are important. We at Columbia Data Products do not dispute that. They account for a sizeable portion of your command processing latency. You should not, however, pay a premium for a "fast" access time if you can't take advantage of it. This test will help you determine that. The last example I wish to give will compare two hard drives. The first hard drive has a 1 millisecond average access time and the other has a 100 millisecond average access time. The 1 millisecond access time hard drive can move data off itself and into the computer at 100 thousand bytes per second (100 KBS). The 100 millisecond access time hard drive can move data off itself and into the computer at 900 thousand bytes per second (900 KBS). To move 900 thousand bytes, the 1 millisecond access time hard drive takes 9 seconds while the 100 millisecond access time hard drive takes only 1 second. Therefore, the 100 millisecond access time hard drive is 9 times faster moving data even though it is 100 times slower moving its head! It probably costs a lot less, too. I think you are beginning to see the picture. RUNNING TESTDISK NOTE: TESTDISK WILL NOT RUN WITH DOS VERSIONS 3.31 OR GREATER. THE I/O INTERFACE THAT TESTDISK USES TO MEASURE DISK PERFORMANCE HAS BEEN CHANGED WITH THESE DOS VERSIONS. A FUTURE RELEASE OF TESTDISK WILL WORK WITH THESE DOS VERSIONS. To start testdisk, simply type TESTDISK and press enter. First, you must choose whether or not you want to run testdisk in color. Next, you will be asked whether or not your printer can print the two characters which are displayed on the screen. A printer which can print the IBM character set will have no trouble printing the displayed characters. The printer will print out a graph of the test results, if you desire, at the completion of the test. You will then be asked for a DOS drive letter (C:, D: etc) to test. Next you will be asked how much data in Kilobytes you would like to read. We recommend a minimum of at least 1000K for an accurate test. The test then proceeds to perform a verification pass to insure that the area being tested is error-free. If it is not, an error will occur and the test will stop. If it is error-free, the test will continue. The data is then read by two methods--SEQUENTIAL and REPETITIVE. The SEQUENTIAL test will read data from the drive the same way that your application program will. The REPETITIVE test, which reads data the way most "disk test" programs do, will not reflect real world performance. We have included the REPETITIVE test for comparitive purposes only. TESTDISK creates a graph of 16 individual tests, 8 SEQUENTIAL, and 8 REPETITIVE. Each individual test reads data in only one block size. The tests are run in pairs (one SEQUENTIAL and one REPETITIVE) for block sizes of 1/2K, 1K, 2K, 4K, 8K, 16K, 32K and 64K. In other words, if you are reading 1000K in 1/2K blocks, you will access the disk drive 2000 times for each individual test. For the 1K test, you will access the disk 1000 times, then 500 times for the 2K test, 250 times for the 4K test, and so on. The SEQUENTIAL test reads the disk one sector after another, beginning with data in sector 1 on the hard drive. To read data sequentially, the hard drive head reads all the sectors on the first track, then is moved from that track to the next track, and so on, until the amount of data requested is read. The first sequential test shows the rate for reading the requested amount of data in 1/2K blocks, starting in sector 1. The second test also begins in sector 1 and reads in 1K blocks, the third begins once again in sector 1 and reads in 2K blocks, and so on. The REPETITIVE read test is included to show how most other disk test programs read their data--repetitively, 64K at a time. Rather than reading the data sequentially, as required by most application programs, data from the same spot on the disk is repeatedly read over and over in 64K blocks. Reading the same data repeatedly and in such large blocks does not accurately test disk performance for an application program. Our first repetitive test represents the requested amount of data being read from one spot on the disk in 1/2K blocks, the second test shows the rate for reading the same data from the same spot in 1K blocks, and so on, up to 64K. As you may have noticed, when running TESTDISK there is a lot of descriptive text that is printed to the screen to inform you of what is happening. After running the test a few times all this descriptive text may become a bit annoying. This is why we have included the ability to turn all the descriptive text off. Simply type TESTDISK /I when loading TESTDISK and the descriptive text will not appear. IN SUMMARY The sequential test results will indicate your disk performance on user programs which call for data from the disk. Use of the repetitive test will show only what other test programs show. HAPPY TESTING!!