Rick Fochtman said:
Ken, the physical BLKSIZE of the directory is 255, plus a 8-byte physical
key.
I think you'll find that's 256.
From: Rick Fochtman <[EMAIL PROTECTED]>
Reply-To: IBM Mainframe Discussion List <IBM-MAIN@BAMA.UA.EDU>
To: IBM-MAIN@BAMA.UA.EDU
Subject: Re: Top 10 software install gripes
Date: Sat, 12 May 2007 10:06:29 -0500
-------------------------<snip>-------------------------
The PDS directory records are a fixed size, LRECL=255, I believe. My reason
to allocate a PDS filling a track, or multiples, with the directory comes
from thinking half-track blocking is efficient for reads and if I use one
half, or full track directory then I am doing the least amount of I/O as
possible. Reality may be different. The optimum number of PDS entries to
allocate depends on the type of PDS, loadlib or other. For other, are
statistics kept or not. How many of us ignore taking the time to plan the
need and just allocate with a larger number than we need? I chose to
allocate larger based on number of tracks. I no longer use a PDS for
non-loadlib datasets.
--------------------------<unsnip>-----------------------
Ken, the physical BLKSIZE of the directory is 255, plus a 8-byte physical
key. You can set a PDS BLKSIZE to any value you like, but it will have NO
effect on the directory characteristics. On a 3390, that means that you
will always get 45 directory blocks to a track, or 44 blocks plus the EOF
mark.
-------------------------<snip>--------------------------
I believe in my testing I found 44 directory blocks will fit on one track
with the end-of-file block following it on the same track. There will be
room for the starting text of one member on the same track. 45 directory
blocks will fit on one track with the end-of-file block spilling over to
the second track. For this reason I had been using multiples of 44
directory blocks when I used a PDS. When you use ISPF to access a member
list, you must read all of the necessary directory entries to populate the
list. 44 means reading two half track blocks, while 45 means reading three
to reach the end-of-file block. That was from m belief the directory
entries fit in the half-track blksize I used to allocate the PDS.
-----------------------<unsnip>------------------------
You still do the same number, 45 reads (assuming no CCW chaining) for each
track of directory space. See above.
------------------------<snip>--------------------------
Nowadays I would still consider using 44 as my smallest, and if I need more
I would use multiples of 45 plus 44 for the last.
------------------------<unsnip>------------------------
I do the same, partly because I'm just an old-fashioned cuss.
-------------------------<snip>---------------------------
Conclusion - vendors should allocate a directory to fit what they deliver
plus maybe 10% for future growth.
-------------------------<unsnip>-----------------------
Make that 20% and round to a full track and I'll go along with you. There
may be a few poor slobs stuck on physical 3390's yet and in that case,
there are RPS considerations.
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