Top Document: comp.arch.storage FAQ 1/2 Previous Document: [6] Disk Next Document: [6.2] Optical {Brief} See reader questions & answers on this topic! - Help others by sharing your knowledge From: Disk Many disks (hard, floppy and optical) run in CAV (Constant Angular Velocity) mode. In this case, the disk spins at a constant rate, and there are the same number of sectors per track on inner and outer tracks. This means that the bits are farther apart on the outer tracks, potentially wasting space. The transfer rate is constant, as the number of bits/track is same and the time/track doesn't vary. CDs (and video laser disks, I believe) and early Macintosh floppies run at Constant Linear Velocity (CLV). That is, the bits are all roughly the same size, and the rotations per minute of the drive is adjust as the head moves in and out. This gives the best areal density of bits, at the sacrifice of seek speed, since every seek requires an adjustment of the rotation speed. The transfer rate is constant, as the size and spacing of bits is constant and the linear velocity is constant. The current rage is ZCAV, Zoned Constant Angular Velocity. Most modern SCSI disks have this feature, and the newest MO drives do, as well. There are a number of zones defined on the disk. The number of sectors per track is different in each zone. Thus, the data is packed more densely than normal CAV, but seek speed is not sacrificed. Another effect of ZCAV is that the media transfer rate varies depending on the head position, because the time/track is constant and the bits/track vary; for example, the Seagate ST12450W Barracuda drive varies from 68 to 113 Mbits/sec, almost a factor of two different. http://perspolis.usc.edu/Users/shkim/dblab_papers.html has a couple of papers on this topic, and I (rdv) have a paper in consideration for a conference on the topic (6/96). User Contributions:Top Document: comp.arch.storage FAQ 1/2 Previous Document: [6] Disk Next Document: [6.2] Optical {Brief} Part1 - Part2 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: rdv@alumni.caltech.edu
Last Update March 27 2014 @ 02:11 PM
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