SUBSYS DD Parameter › Coding the SUBSYS JCL Parameter › MVS SUBSYS Restrictions and Special Processing

MVS SUBSYS Restrictions and Special Processing

MVS imposes some restrictions and special processing rules for the use of the SUBSYS DD parameter. These restrictions and special processing rules are documented in the IBM JCL Reference Manual and are listed below for your convenience:

• The SUBSYS DD statement parameter cannot be coded for an SMS-managed data set. However, with the CA Compress subsystem's associated DDNAME feature, you can circumvent this IBM restriction. The associated DD statement referenced using the addname subparameter on the SUBSYS parameter can specify an SMS-managed data set.
• The SUBSYS DD statement parameter cannot be coded with SYSOUT=, *, DATA, DDNAME DLM, DYNAM, OUTPUT or QNAME.
• When the SUBSYS DD statement parameter is coded, the parameters COPIES, DEST, FCB and OUTLIM are ignored.
• If DUMMY is specified, the SUBSYS parameters are checked for syntax, and if they are acceptable the data set is treated as a dummy data set.
• If SUBSYS= is specified on an overriding DD statement, a DUMMY parameter on an overridden DD statement is nullified, and a UNIT parameter is ignored.
• SUBSYS does not work correctly with System Determined Blocksize (SDB). SDB is an IBM facility which chooses the optimum BLKSIZE for a data set based on its device type when the user codes BLKSIZE=0 in JCL. Because a SUBSYS data set looks to the system like a SYSIN/SYSOUT data set, SDB computes the BLKSIZE by adding 4 to the LRECL instead of choosing the BLKSIZE appropriate to the actual device. This results in very poor blocking factors and defeats the purpose of compression.