🚀 go-pugleaf

In article <2474@peora.UUCP>, joel@peora.UUCP (Joel Upchurch) writes:
> 	        ...WORM optical disk...
> 	                                         ...would not make good
> 	paging devices because of the awful seek times and rather anemic
> 	transfer rates associated with these devices. Has anyone seen
> 	any optical disks with numbers in these categories comparable
> 	to good magnetic disks?

Part of the problem is the same thing that makes Mac floppies slow.
CD optical disks are specified to have a constant linear velocity of
material passing under the heads.  This means that the disk must
spin faster and slower as the head seeks in and out.  While they could
make the heads seek as fast as magnetic disks, they just can't spin
the disk twice as fast, or brake to half the speed, in a reasonable time.

I don't know if WORM disks are done this way.

Traditional magnetic disks are spec'd at a constant rotational velocity
(e.g. 3600 RPM), which makes the spinning and the seeking independent.
This causes the bits on the outside of the disk to be "wider" than the
bits on the inside, since more material zips under the head in the
same amount of time.  Since it must be able to read and write bits
anywhere on the disk, it has to be good enough to do it on the inner
tracks where the bits are smaller.  But when on the outer tracks, all
that precision goes to waste.

I don't understand why nobody has built magnetic disks that spin at
a constant speed, but vary the clocking of data to the disk so that
all the bits end up the same width on the media.  This means that you
might get 30,000 bytes per track on the inside and 90,000 on the 
outside -- but who cares?  On a SCSI interface, the system doesn't know
where the tracks and cylinders are anyway.

I don't know how to figure it out exactly, but I suspect that this
simple change (to a disk and its controller) could double the amount of
stuff you could put on the same disk with the same heads and almost the
same electronics.
-- 
John Gilmore  {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu   jgilmore@lll-crg.arpa
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