> On Feb 1, 2022, at 12:21 PM, Paul Koning via cctalk <cctalk@classiccmp.org>
> wrote:
>
>
>
>> On Feb 1, 2022, at 12:16 PM, Mike Katz via cctalk <cctalk@classiccmp.org>
>> wrote:
>>
>> In the rotating drive world there is constant linear velocity (CLV) and
>> constant angular velocity (CAV) drives.
>>
>> On CLV drives the speed of rotation would vary based on the track (slower in
>> the inner tracks and faster on the outer tracks). This meant that the data
>> rate and number of bits/track remained constant.
>
> Slower on the outer tracks, I believe. CDs work this way.
More precisely:
CLV means slower rotation when positioned on the outer cylinders. The outer
cylinders have more sectors; the layout is such that the linear bit density is
roughly constant, which in turn because of the constant linear velocity means
constant data rate.
>> On CAV drives the rotational speed of the drive doesn't change, this means
>> that the data rate and number of bits/track changes depending on the track.
>
> It means that only if the sector count changes. That's true for modern
> drives and for the CDC 6603; it wasn't true for quite a while. A lot of
> "classic" disk drives have constant sector counts. So, for example, an RP06
> is a CAV drive and its transfer rate is independent of cylinder number since
> the sector count per track is constant.
>
> I think hard drives are CAV as a rule because changing the spin rate as part
> of a seek takes too long.
Variable sector count is independent of CLV vs. CAV. Modern drives have it,
classic CAV drives mostly do not. A CAV drive with fixed sector counts has
fixed data rate; a CAV drive with more sectors on the outer tracks has higher
transfer rate on those tracks.
paul
