On 02/12/2010 11:25 AM, Guy Gardoit wrote: >> Does "wasted space" in PDSE's really matter all that much? I'll bet no >> one has al their PDS data set compressed 100% of the time - that's called >> "wasted space" not to mention the constant battle with directory blocks. >> PDSE's are not perfect but this stuff about "wasted-space" is just hot air >> AFAIC >>
As with all such questions, "it depends". Some large PDSs may have low update activity, so that tuning directory space and over allocating for deleted "gas" are not issues and space usage by the PDS is very efficient. Some PDSs are large enough that consistently finding enough free space on a single 3390-9 to allocate a new copy may be problematic (and going to larger devices may not yet be practical). The additional space to convert this to a PDSE can then be a major issue, as even without a need to compress or resize the library, upon occasion one will need to allocate a new copy to RESTORE backups or RECALL migrated versions. If in addition the large PDS contains many very short members, the size increase for the PDSE can be very substantial, perhaps exceeding the capacity of a single drive, making allocation impossible. That said, we still have cases where a PDSE is clearly the best choice, mostly in cases where having to take down applications to resize or compress a library would be a major issue. We even had one bizarre application in which we chose to use a PDSE even though the required file size went from 5 cylinders to over a 100, because the PDSE was more reliable. This was an ISPF Table library and at the time we had an ISPF application which updated a fairly large table 100's of times in the course of a day (and also many smaller tables with a similar update rate). Because of the way ISPF allows for table padding, on a PDS many of these updates were "in place". Every couple of months, at a time when the system was maxed out and TSO slowed down, someone would think their TSO session was hung and cancel it in the middle of a large table update and totally trash the table. With a PDSE, there is no update-in-place, so while a cancellation might lose an update, the table itself would not be corrupted. Using a PDS the physical size of the library stabilized at under 5 cylinders. We were surprised to find that as a PDSE the table grew to many extents and over 100 cylinders before it stabilized! It appears that ISPF leaves table library member references hanging after use in such a way that the PDSE was in many cases unable to reuse the deleted space until the TSO/ISPF session ended. Near the end of a working day we would see the in-use space on the ISPF table PDSE approach 100+ cylinders, and then once all users had logged off it would drop back down to around 1 cylinder in use, and repeat the pattern the next day. -- Joel C. Ewing, Fort Smith, AR [email protected] ---------------------------------------------------------------------- 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
