Re: ancient disks, Architectural implications of locate mode I/O

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.

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