Best IAM global options settings

Except as recommended here or elsewhere, such as in Setting the IAM Global Options, it is highly recommended that you keep the default values for the IAM Global Options. If there is a desire to change beyond what is recommended, please contact BMC Support for advice. Some of the default values are in the list because we have seen customers change them and then encounter issues by so doing.

• Keep DSORG=PS set. The use of DSORG=DA has caused problems with various data set management utilities and is therefore not recommended. FDREPORT is a BMC solution that makes it simple to find IAM data sets on your DASD volumes.
• Keep BELOWPOOL=YES set. Customers have used it for several years now without problems, and it is essential, particularly for keeping below-the-line storage at a minimum under CICS.
• Keep the ENHANCED option enabled. IAM Enhanced format files offer significant advantages over the original COMPATIBLE format files, including the ability to take extents and increase the size of the overflow area as the file grows, buffering enhancements including significantly more buffers, and use of data space or 64-bit virtual for index, to name a few.
• Do not reduce the default values for BUFSP or CICSBUFSP except on the recommendation of or consultation with BMC Support. In fact, with the changes to real-time tuning to not use up all of the available storage, you may be able to increase the CICSBUFSP value. One of our customers recently did that in small increments over time and had excellent results. You may also want to consider using 64-bit virtual storage for the buffers, particularly for CICS and IAM/RLS. An increase in the MEMLIMIT value (specified as a JCL parameter on the EXEC card for the relevant job step) of at least 2 gigabytes will be necessary if that is done.
• Setting SMF=YES and RECTYPE=201 (or another number if 201 was previously in use) is highly recommended. This enables the collection of the IAM SMF records so that IAMINFO reports can be produced from the SMF data if needed without having to include an IAMINFO DD card in your JCL.
• If using ESDS files with IAM Data Compression that will be updated, it is recommended to provide some free space in case an updated compressed record increases in size. This is done by setting ESDSINT=n. Suggested values are 1 thru 5.
• Setting either ENABLE=XESDS or ENABLE=PSEUDO is recommended if you are using ESDS files to avoid the 4 gigabyte limit on the amount of data in an ESDS caused by using the standard 4-byte RBA value.
• Set ENABLE=EAV if you have EAV (Extended Attribute Volumes – the super large volumes). This will cause them to be automatically eligible for EAV use without having to code EATTR(OPT) on the DEFINE CLUSTER.
• If you prefer to use the IBM Hardware Compression instruction over the IAM software compression, then set COMPRESSTYPE=HW.
• To maximize CPU time savings, you can turn off automatic IAM data compression by setting DATACOMPRESS=99999999 (eight 9’s).
• To use 64-bit buffering as the default set ENABLE=BUF64 and CRBUFOPT=64BIT. At this time (as of z/OS 2.2) there is some increase in CPU time caused by the page fix process of 64-bit virtual storage. If your use of IAM is primarily for reducing CPU time, then we do not recommend setting those values. You may want to use the 64-bit buffering for address spaces such as CICS FOR’s or IAM/RLS or IAM/PLEX, which can be done via the IAM Overrides.
• Using RLS=(OPTIONAL,TABLE) is recommended rather than (REQUIRED, TABLE). The latter requires that the IAM/RLS or IAM/PLEX data set name tables be loaded by starting either an IAM/RLS or IAM/PLEX address space before any IAM file can be opened, even if they do not require IAM/RLS or IAM/PLEX.