[Info-vax] disk sizes

[Hans Vlems]

On 20 feb, 15:48, hel... at astro.multiCLOTHESvax.de (Phillip Helbig---
undress to reply) wrote:
> In article <ijr7p5$un... at Iltempo.Update.UU.SE>, Johnny Billquist
>
> <b... at softjar.se> writes:
> > On 2011-02-20 14:57, Phillip Helbig---undress to reply wrote:
> > > Standard hard-disk sizes are 1.07 GB, 2.1, 4.3 (or sometimes the more
> > > precise 4.29---one rarely sees the more precise 2.15).  8.6 (8.59) is
> > > also relatively common.  This is clear, since 1024^3=1073741824 and then
> > > one has 2, 4 or 8 times this.  So far, so good.  After that, sizes tend
> > > to be 9.1, 18 (18.2---though 17 (roughly twice 8.6) also exists), 36
> > > (36.4), 73 (72.8) and 146.  These are also powers of two times the new
> > > basic size, but where does the 9.1 basic size come from?
>
> > Are you aware that disk manufacturers tend to give disk sizes in powers
> > of 10, not power of 2, as this makes the disks look a bit larger?
>
> Right, that is why 1-Gigibyte is marketed as 1.07 Gigabyte etc; see
> above.  My question is why the jump from 8.6 to 9.1?
>
> > Besides, the disk geometry and sizes almost never means you end up with
> > something even divisble by any "sensible" numbers, what are you trying
> > to figure out here?
>
> I just want to understand it.  :-|

One reason for this is the way disks are sectored. On older disks, say
an RA82, the inner tracks have the same number of sectors (or blocks,
whatever) as the outer tracks. So bit density is a lot higher on the
inside tracks.
So outer tracks go more sectors. At that point there is no direct
relationship any moer between the tracks and the sectors on a platter.
It may, perhaps, explain the increase in disksize.
Hans