spp@oz.berkeley.edu (Steve Pope) (11/17/86)
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)
dmc@videovax.Tek.COM (Donald M. Craig) (11/19/86)
Fresh from Comdex, here are the specifications on the new (no prices yet) 6.8 GByte Kodak double sided write once 14" optical disc drive. It is organized in five constant speed bands, meaning you have to change the angular velocity of the disc when you switch from band to band. There is also a concept called a 'window', which essentially refers to the ability of the tracking mechanism to select one track out of some. Drive: Capacity Per Window Per Band Band 0 2.1 MByte 1.24 GByte Band 1 1.7 MByte .86 GByte Band 2 1.4 MByte .61 GByte Band 3 1.2 MByte .41 GByte Band 4 1 MByte .28 GByte Capacity per side: 3.4 GByte Capacity per sector: 1024 Bytes Access Time within window: 3 ms. Access Time within band average: 100 ms. Access Time between bands average: 400 ms. Recording Density: 21,000 bpi Track Density: 14,111 tpi 24" high, 17.7" wide, 24" deep, 200 lbs. very heavy 90 to 127 VAC, 6.0 Amps. average, 15.0 Amps. max. Controller: SCSI, IPI 3 interface, 1.0 MByte per second transfer rate Burst transfer rate of 10 Mbyte per second 128 KByte buffer in controller 7" high, 17.7" wide, 24.0" deep, 60 lbs. heavy. 90 to 127 VAC, 6.0 Amps. max.