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ILEAVE.DOC
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1988-02-13
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Most hard disks in PCs have an "interleave value" from 1 to 7. This value
represents the spacing between groups of data on your hard disk. These groups
of data are called sectors.
Your hard disk system reads or writes data on the disk itself as the disk
spins. The disk spins at a constant rate. It is similar to the way that a
record player works. The head moves from the edge to the center, and the disk
spins at a constant rate.
Your computer can not always accept (or supply) the data fast enough for the
disk drive. In such a case, the hard disk system causes your computer to wait
until the place for that data comes around again. This takes a little under
seventeen thousandths of a second.
If your computer is not able to accept each sector as it comes by, you will
notice that the performance of your system is slow. To improve its
performance, computer manufacturers shuffle the sectors so that the first one
that is requested is never located next to the second one, the second one is
never next to the third one, and so on.
You could visualize this by cutting a pizza into seven slices. Using a tag,
number each one. Now change their positions so that none is next to the piece
that it originally had as a neighbor. If you evenly shuffled them, you might
have the arrangement of "1-5-2-6-3-7-4". In this case, the interleave factor
is two, since the distance from piece number 1 to piece number 2 is two. Right
after you cut the pizza, when they were numbered "1-2-3-4-5-6-7", the
interleave was one. If you arranged the pieces "1-6-4-2-7-5-3", the interleave
would be three.
The hard disk interleave is performed just like this, but with seventeen slices
instead of seven. As the distance between numbered sectors increases, your
computer has more time to process the data in each sector.
The manufacturer finds the point where there is enough time to process each
sector, but not so much time that the computer is waiting too long for each
sector to arrive at the read/write head.