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INTLEAVE.DOC
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1987-05-04
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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