Libris Britannia 4/science library(b).zip/science library(b)/INFO/DISK_ART.ZIP/DISKS2

Text File | 1989-06-09 | 17KB | 297 lines

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        ║                                                              ║
        ║                       A Hard Disk Drive                      ║
        ║                              for                             ║
        ║                     Steve's Dream Machine                    ║
        ║                                                              ║
        ║                              by                              ║
        ║                         Steve Gibson                         ║
        ║                  GIBSON RESEARCH CORPORATION                 ║
        ║                                                              ║
        ║     Portions of this text originally appeared in Steve's     ║
        ║               InfoWorld Magazine TechTalk Column.            ║
        ║                                                              ║
        ╚══════════════════════════════════════════════════════════════╝



        I love hard disk storage, it's elegant, amazing, tricky,
        logical, and completely understandable. So let's begin by
        discussing one of my favorite aspects of modern personal
        computer architecture, and some critical components of Steve's
        Dream Machine... the Hard Disk Storage Sub-System.

        We all want several things from our hard disk systems: High
        Speed, High Capacity, Low Cost, and High Reliability. I've found
        a unique combination of hard disk and controller, for any
        machine with a 16-bit I/O bus, which delivers all four in
        spades.

        The performance of a hard disk system is determined by two
        simple and separate things: The average time required to begin a
        data transfer and the speed of that transfer once it begins.

        In my opinion the world is completely seek-performance crazy.
        When someone asks "How FAST is that drive?" they're speaking
        only of the average seek performance. Sure it's a factor, but
        it's FAR from being the most important issue. What matters much
        more is the CONTROLLER's data encoding format, minimum
        achievable sector interleave, head switching behavior, and
        believe it or not, the number of heads on the drive!

        DOS numbers a disk's sectors sequentially from the outside
        inward. When it wants to read or write a sector, it first
        determines where the sector is located on the drive then sends
        the heads to that location. This means that the issue is not
        how long it takes a drive to move its heads to cylinder 100, but
        rather how long it takes to move them to SECTOR NUMBER X.  For
        different drives these can be very different questions.

        For example, let's take the ubiquitous Seagate ST225 20 megabyte
        hard disk drive as our baseline. It can't handle RLL encoding,
        so it's limited to 17 sectors per track. It also has four heads
        for four tracks per cylinder. Therefore this drives has a
        CYLINDER DENSITY of 17 times 4, or 68 sectors per cylinder.

        Now let's compare this with the Steve's Dream Machine drive, the
        MiniScribe 3650. This lovely half-height drive handles RLL
        encoding without a hiccup for 26 sectors per track, and its 6
        heads combine to deliver a cylinder density of 156 sectors per
        cylinder.

        In other words, the 3650 packs 2.29 times more sectors into each
        cylinder than the ST225. DOS's sector numbering scheme means
        that the 3650 needs to move its heads 2.29 times less far, or
        about 44% the distance of the ST225!

        So while the Miniscribe drive might appear to be slow, with its
        head positioner rated at 61 milliseconds average access time, if
        we compare apples to apples, using the ST225's 65 millisecond
        speed as a reference, the 3650 is equivalent to a ST225 drive
        with a 26 millisecond actuator!

        In order to correctly compare hard drive access times, I
        designed an index which takes all of these factors into account
        and which can be used to correctly rate any drive. I call it the
        Real Sector Access Factor, or RSA Factor.

        To determine it for any drive simply multiply the sectors per
        track (17 for MFM encoding, 26 for RLL) by the drive's head
        count, then divide by the drive's average seek time. This yields
        an index which is completely compensated to account for cylinder
        density and allows drives to be correctly compared.

        The RSA Factor for the ST225 is 1.04, versus 2.55 for the
        Miniscribe 3650. The Seagate ST238 with its RLL encoding comes
        in with a 1.60 and the ST251 with its 40 millisecond average
        access ranks an RSA Factor of only 1.70. As these numbers
        demonstrate, it's important to compare apples to apples when
        evaluating drive specifications. The "sluggish" 3650 even beats
        out the "swifter" ST251 when compared correctly.

        In the case of average sector access times, the actual distance
        the heads must move is really determined by the number of
        sectors the drive and controller are able to stuff onto each
        cylinder, not by shaving milliseconds from average access times.

        The Miniscribe 3650 is not quite officially RLL certified,
        though I hear rumors that it's about to be, simply because it
        works so well. I've tested many of them myself, and the bright
        boys at Northgate Computer Systems (who turned me on to this
        drive in
        the first place) are shipping thousands with RLL controllers in
        their 286 AT compatibles. They've had no problems. I'm quite
        comfortable with the 3650 and RLL encoding.

        Finally, the 3650 is rated as having 809 cylinders, though it
        actually has 852. I've been low-level formatting mine out to 842
        cylinders. Then, under DOS 3.3 with RLL encoding, you get two
        MAXIMUM SIZE 33.4 megabyte DOS partitions! They couldn't be any
        bigger! Sixty-seven fast megabytes in an inexpensive half-height
        drive is hard to beat!

        Okay, so we've defined the real performance of a hard disk sub-
        system to be: The average time required to begin a data
        transfer, and the time required to preform the transfer once it
        has started. We then examined the first of these terms and saw
        that the data encoding technology (MFM or RLL) and the drive's
        head count both dramatically affect the system's actual head
        seek performance since they determine the average distance the
        head must move to get to the proper DOS sector. Now we'll examine
        the second determiner of hard disk system performance, the actual
        data throughput.

        Many tricky and interacting issues determine a hard disk
        system's delivered data throughput, but none of them are very
        tough to understand.

        The raw data that rotates underneath our hard disk's heads
        moves at quite a clip. Data bits that are encoded with Modified
        Frequency Modulation (MFM) technology flow to and from the
        drive's head at 5 million bits per second, and Run Length
        Limited (RLL) encoding moves its data at 7.5 million bits per
        second. After subtracting the inter-sector gap intervals and
        sector addressing overhead, this translates to 522,240 bytes of
        real data per second for MFM and 798,720 bytes per second for
        RLL.

        Unfortunately the hard disk controllers and motherboards used in
        PC, XT, and most current generation AT computers are completely
        unable to keep up with data flowing at this rate. So the
        practice known as SECTOR INTERLEAVING was invented to slow
        things down to a rate which our computers can handle. Sector
        interleaving spaces successively numbered sectors out around the
        disk so that our slower hard disk controllers and computers can
        digest the prior sector before the next one begins. Failing to
        space the sectors far enough apart incurs the substantial delay
        of waiting for the disk to spin all the way around again.

        The original IBM XT's hard disk was interleaved at 6-to-1 (6:1)
        which meant that 1/6th of the track's sectors were read during
        each revolution of the disk and that six revolutions were
        required to read a single 17-sector track. This also meant that
        the original XT's effective data transfer rate was 522,240
        divided by 6, or 87,040 bytes per second. Not very exciting.

        Even today things are frequently not much better. I have upset
        Western Digital in the past by reporting that most of the
        machines I had tested were not fast enough for the default 3:1
        sector interleave they were using on their MFM controller with
        the result that only one sector was being transferred for each
        revolution of the disk. This of course resulted in horrible
        30,720 byte per second throughput. The fact is that most of
        today's XT and AT machines are using MFM encoding with an
        interleave of 3:1 or 4:1 and delivering unexciting throughputs
        of 174,080 or 130,560 bytes per second respectively.

        When I wrote a series of columns on hard disk performance, I
        reported that RLL encoding was "not here yet" but that I was sure
        it would be a good thing and that we were only premature, rather
        than wrong, about its ultimate viability.  Well, I'm delighted to
        report that RLL encoding is FINALLY
        REALLY HERE! The controllers have their acts together and
        reliable and robust RLL drives are readily available. If
        horrible experiences set you forever against RLL, I strongly
        advise you to re-address the issue. As long as you
        choose your drive and controller carefully, you won't have any
        trouble.

        Aside from cramming more data into a drive, RLL also increases
        the real seek performance of any drive. Remember our discussion
        of Real Sector Access (RSA) Factor. Raising the drive's cylinder
        density by 150% drops its average seek times to just 66% of what
        they would be with MFM encoding. And since the drive's data is
        encoded at 150% density, the raw data rate from the drive is 150%
        higher.

        However, a higher data rate from the drive doesn't help us much
        if we must immediately water it down with a large sector
        interleave. Western Digital's latest 1002A-27X 8-bit RLL
        controller defaults to an unexciting interleave of 4:1,
        delivering 199,680 bytes per second throughput which beats an
        MFM controller with 3:1... but not by much.

        The great news is that we're just beginning to see some really
        hot (and inexpensive) hard disk controllers which are fully able
        to keep up with a 1-to-1 interleaved disk for the delivery of
        screaming 798,720 byte per second data transfer rates! That's
        just shy of 0.8 megabytes per second!


        I've explained my choice of hard disk drive for Steve's Dream
        Machine. The Miniscribe 3650 is very inexpensive (several booths
        at a recent Southern California swap meet were selling them for
        between $290 and $300), it's half height (so you can have a pair
        of them!), utterly capable of handling RLL encoding, and places
        six heads under the control of a 61 millisecond (average seek)
        stepping motor positioner.

        Twenty-six sectors per track and six tracks per cylinder give the
        3650 a cylinder sector density which is 2.29 times higher than a
        typical four head MFM drive, so it actually performs like a
        drive with a 26 millisecond average seek time because the heads
        only need to move 44% as far to get to the same sector.

        Even though Miniscribe says the drive has only 809 cylinders it
        actually has 852 physically and I've been formatting all of mine
        out to 842. Northgate Computer accepted my suggestion and has
        been doing the same to hundreds of theirs also without hitch, so
        I'm quite comfortable suggesting this to everyone.

        I run under DOS version 3.3 because it's able to split the drive
        into two MAXIMUM SIZE 33.4 megabyte partitions WITHOUT the need
        for any messy third-party partitioning software. This yields a
        "C" and "D" partition of 33.4 megabytes respectively or 67
        megabytes overall!

        So what about a hard disk controller? Well in this day and age
        there's no excuse for NOT going with RLL and a 1:1 sector
        interleave. So let me make this point quite clear. First, even
        though disks seem to be spinning quite fast, they're really
        quite slow. 3600 RPM is only 60 revolutions per second, which is
        16.67 milliseconds per revolution.

        Now imagine that we wish to read or write a moderate size file
        of 26K bytes. Since sectors are 512 bytes, 26K bytes requires 52
        sectors. On an MFM format drive with 17 sectors per track this
        fills 3 tracks. A typical interleave of 4:1 requires 12 disk
        revolutions, for a total transfer time of 0.2 seconds. However
        an RLL controller with 26 sectors per track and 1:1 interleaving
        moves the same 52 sectors in just two revolutions or 0.033
        seconds. Two revs versus twelve... or SIX TIMES FASTER!

        I'm delighted to tell you that choosing a hard disk controller
        was quite simple, because nothing even comes remotely close to
        Adaptec's model 2372 masterpiece. In the first place, it REALLY
        handles a SUSTAINED 1:1 interleave. Other 1:1 controllers may
        grab an entire track in one revolution, but they're then unable
        to continue with the next track immediately afterward.
        Consequently the system's performance drops by half to that of a
        2:1 interleaved drive. The Adaptec sustains 798K bytes per
        second across multiple tracks.

        Secondly, you don't need a 16 megahertz 386 system. Any AT
        compatible can achieve screaming 800,000 bytes per second
        transfers with this controller. It comes in two flavors, the
        2372 handles two hard drives as well as two high or low density
        floppy drives and the 2370 just handles two hard drives.

        The built-in low-level formatting software has to be seen to be
        believed. It's the cleanest and most comprehensive of any I've
        ever seen. If you want to run with multiple partitions, or a
        partition larger than 33 megabytes it will actually create the
        required CONFIG.SYS driver by "downloading" it from its own ROM
        onto the root directory of the hard disk! Unbelievable.

        Finally, and most incredibly, it is so compatible with the
        standard AT hard disk MFM-style chip sets that it DOESN'T
        REQUIRE ANY ROM BIOS WHATSOEVER up there in the high memory
        "twilight zone!" After booting and initializing itself, the ROM
        is never again used. This means that the "twilight zone" region
        is not reduced in size and fragmented. Then utilizing Steve's
        Dream Machine's memory manager, 386-to-the-Max, 225K of
        completely free contiguous "twilight zone" memory is available
        for loading TSRs and other resident software!

        Finally, by using a non-RLL capable Seagate ST225 drive and some
        ruthless worst-case data pattern testing software I've
        developed, I was able to quantitatively compare the robustness
        of the RLL data separators used in all of the contending
        controllers. The Adaptec 2372 is absolutely up at the top of the
        heap of RLL reliability because it makes the Seagate ST225,
        which is totally worthless for RLL in any case, look BETTER than
        any of the other RLL controllers do. So I'm more confident of
        the Adaptec with a real RLL drive than I would be with any of the
        others.

                                   - The End -


                     Copyright (c) 1989 by Steven M. Gibson
                             Laguna Hills, CA 92653
                            **ALL RIGHTS RESERVED **