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.
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. 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. Couple that with having learned CYL allocation performed better than TRK or BLK and assigning one track to a small PDS is of no concern. I still favor allocating based on some planning, but laziness prevails more often than not. Take a bunch of so-called facts and we end up doing things of dubious quality. And as things change we persist in using our outdated thinking. Conclusion - vendors should allocate a directory to fit what they deliver plus maybe 10% for future growth. >>-----Original Message----- >>From: IBM Mainframe Discussion List [mailto:[EMAIL PROTECTED] >>Behalf Of Tom Marchant > >>> >>>I'm not sure how many PDS directory blocks will fit on a track, but I >don't >>>see >>>why it's important either. IIRC, the remainder of the last directory >track >>>will >>>be used for data. In any case, I see no reason to make the directory >larger >>>just to make it fill the last track. > ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
