[Info-vax] disk sizes

[Johnny Billquist]

On 2011-02-20 16:17, Hans Vlems wrote:
> 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.

Damn! And you got off to such a good start. :-)
The RA82 definitely have different numbers of sectors depending on where 
it is on the disk. And even worse. One sector per track is reserved as a 
replacement block. And then you have a few tracks that are totally 
reserved as replacement blocks. And then you have the replacement block 
list, which also lives on the disk, and the service tracks, which are 
used purely for tests.

By "older" disks, you should have suggested something like an RP07 or 
something, where you actually addressed the disk by track,surface,sector.

> 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.

Right. But what Philip should also think of, is that on any particular 
track, you do not have a power of 2 numbers of sectors. So the disk 
geometry might look like 2568 tracks, 17 sectors per track, and 7 
surfaces. Now, how you could even hope to get any disk capacity that was 
nicely matched as a power of either 2 or 10 is beyond me. Disk 
capacities are basically just how much the manufacturer can manage to 
squeeze in there.
It follows no arithmetic "rules".

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol