Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rutgers!sri-unix!hplabs!ucbvax!oz.berkeley.edu!spp From: spp@oz.berkeley.edu (Steve Pope) Newsgroups: comp.periphs Subject: Re: Why optical disks are slow to seek; Message-ID: <16395@ucbvax.BERKELEY.EDU> Date: Sun, 16-Nov-86 19:26:35 EST Article-I.D.: ucbvax.16395 Posted: Sun Nov 16 19:26:35 1986 Date-Received: Mon, 17-Nov-86 02:02:43 EST References: <1128@tekig5.UUCP> <5100141@ccvaxa> Sender: usenet@ucbvax.BERKELEY.EDU Reply-To: spp@oz.berkeley.edu.UUCP (Steve Pope) Organization: University of California, Berkeley Lines: 34 Because of a few statements made in the popular media, optical disks have acquired the reputation of being "slow to seek". While this is basically true of a few currently available models, don't expect it to be a permanent, universal problem. Magnetic disks were pretty slow to seek at the corresponding stage in their development. One reason they don't spin the disk faster is because this would increase the energy needed to write bits on the disk (you'd have less time to burn a bit). This would increase the cost of the laser -- under current economics. Another reason is, the data rate is already about the same as magnetic disks, and thus about the same as what current interfaces can handle. Most optical disks have a raw bit-rate between 2 and 25 MHz. The big selling point of optical disks is their capacity, not their access time. Often, there is a single spiral track; others use concentric tracks. Some spiral drives, when they seek, only seek to the approximate vicinity of the desired track, and then spin perhaps several full revolutions before reaching the desired sector. This makes the average seek time a lot higher. However, there's no reason why in the future, optical drives will not be able to seek directly to the desired track. The technology exists. Media defects and resultant sector-redirection schemes also impact seek time and throughput. As a rule, the throughput of an optical disk is much better on long multi-sector accesses than on a series of single-sector accesses for a variety of reasons. It's up to operating systems and applications designers to realize this in order to use the current generation of drives efficiently. steve pope (...ucbvax!spp)