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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: ancient disks, Architectural implications of locate mode I/O
Date: Sun, 7 Jul 2024 06:35:40 -0000 (UTC)
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Stephen Fuld <SFuld@alumni.cmu.edu.invalid> schrieb:
> 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.

They introduced PDSE, see
https://www.ibm.com/docs/en/zos-basic-skills?topic=sets-what-is-pdse
It appears they fixed many of the problems with the original
design, but not all (why is the number of extents still imited?)
It also seems that RECFM=U for load modules is no longer supported,
you have to do something different.