The following explains how a data set is assigned to a particular storage class, as represented on the PMOMON D8 display.
When an SMS-managed data set undergoes an I/O operation, the I/O subsystem obtains the data set storage class through the CASF token located in the DEB (Data Extent Block), which is created at OPEN. There is only one CASF token per DEB, so when a concatenation of mixed-storage class data sets exists, only one storage class, the one belonging to the first data set, is assigned to the DEB. If the first data set is not SMS-managed, the CASF token represents the never-cache storage class.
For example, in the following concatenation, SMS treats each data set as never-cache since the CASF token in the DEB indicates the storage class of the first data set in the concatenation:
//PDS DD DSN=PDS.NEVER.CACHE.CNTL // DD DSN=PDS.MAY.CACHE.CNTL // DD DSN=PDS.MUST.CACHE.CNTL
SMS does not cache these data sets. CA PMO, however, tries to keep these data sets in processor storage, based on BLDL activity.
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