[Info-vax] Example of random access by record number on an RMS fixed record size, relative organization file?

[gah4]

On Thursday, September 21, 2023 at 3:13:00 PM UTC-7, Arne Vajhøj wrote:
> On 9/21/2023 1:10 PM, gah4 wrote:
> > On Thursday, September 21, 2023 at 5:19:50 AM UTC-7, bill wrote: 
> >> While ISAM may be less efficient relative files can waste a 
> >> lot of space. Space is allocated for empty records and back 
> >> in the good ole days disk space was premium and very expensive. 

> > Relative files are convenient if your numbering system is already 
> > not sparse. 

> > Otherwise, it is usually to make it a hash table, and hash the 
> > index before using it to read/write a record.

> A hashing producing non sparse results will typical also 
> produce a lot of collisions requiring collision handling. 

Well, relative lets you figure it out yourself. 

Especially the amount of sparseness you can live with, in exchange
for faster access.  With others, the system figures it out for you.

I believe for IBM ISAM, you have to give the size when you first
create it.  (Actually, that is the usual way files work in OS/360.)

There is a prime area and overflow area.  It seems that it keeps some
space in the prime area for new records, but if they are not uniformly
enough spaced, then they go into the overflow area.  So, not
so different from a hash table.

OS/360 disks are addressed by cylinder, track, and record within the track.
It tries to keep nearby records on the same track, or at least same cylinder.