lotus



previous page: 5.23 DATATAPE DTR-48
  
page up: Storage FAQ Part 1
  
next page: 6.2 Optical

6.1 CAV, ZCAV and CLV




Description

This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.

6.1 CAV, ZCAV and CLV

    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).
    

 

Continue to:















TOP
previous page: 5.23 DATATAPE DTR-48
  
page up: Storage FAQ Part 1
  
next page: 6.2 Optical