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

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)