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From: "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: ancient disks, Architectural implications of locate mode I/O
Date: Sun, 7 Jul 2024 06:23:23 -0000 (UTC)
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John Levine wrote:

> According to Stephen Fuld <SFuld@alumni.cmu.edu.invalid>:
> >> I believe that's what they did with VSAM.
> > 
> > Agreed in the sense that VSAM replaced ISAM, but, and I am getting
> > beyond my depth here, I wasn't aware that PDSs used ISAM.  I had
> > thought they were a thing unto themselves.  Please correct me if I
> > am wrong.  In any event, PDSs in their original form lasted beyond
> > the introduction of VSAM, or the PDS search assist functionality
> > wouldn't have been needed.
> 
> A PDS had a directory at the front followed by the members. The
> directory had an entry per member with the name, the starting
> location, and optional other stuff. The entries were in order by
> member name, and packed into 256 byte records each of which had a
> hardware key with the name of the last entry in the block. It searched
> the PDS directory with the same kind of channel key search it did for
> ISAM, leading to the performance issues Lynn described.


Yes, I don't disagree with any of that.  But I got the impression from
your previous posts that IBM had replaced the search key fields of a
PDS with some kind of VSAM (i.e. b-tree or such) varient, as they did
with ISAM.  If that is true I had never heard about it.



-- 
 - Stephen Fuld 
(e-mail address disguised to prevent spam)