Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-crg!hoptoad!gnu From: gnu@hoptoad.uucp (John Gilmore) Newsgroups: net.arch,net.periphs Subject: Why optical disks are slow to seek; an idea for higher capacity disks Message-ID: <1256@hoptoad.uucp> Date: Tue, 4-Nov-86 04:28:00 EST Article-I.D.: hoptoad.1256 Posted: Tue Nov 4 04:28:00 1986 Date-Received: Tue, 4-Nov-86 10:29:56 EST References: <1128@tekig5.UUCP> <5100141@ccvaxa> <553@cubsvax.UUCP> <2474@peora.UUCP> Organization: Nebula Consultants in San Francisco Lines: 43 Xref: mnetor net.arch:3334 net.periphs:594 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 terrorist, cryptography, DES, drugs, cipher, secret, decode, NSA, CIA, NRO. The above is food for the NSA line eater. Add it to your .signature and you too can help overflow the NSA's ability to scan all traffic going in or out of the USA looking for "significant" words. (This is not a joke, sadly.)