The system can retrieve two or more data sets and process them successively as a single sequential data set. This is called sequential concatenation. The number of data sets that you can concatenate with sequential concatenation is variable. It is governed by the maximum size of the TIOT option. The system programmer controls the TIOT size with the option ALLOCxx member of SYS1.PARMLIB. The smallest TIOT value allows 819 single-unit DD statements or 64 DD statements having the maximum number of units. See z/OS MVS Initialization and Tuning Reference. When data sets are sequentially concatenated, your program is open to only one of the data sets at a time. Concatenated data sets cannot be read backward.

A sequential concatenation can include sequential data sets, PDS members, PDSE members, and UNIX files. With sequential concatenation, the system treats a PDS, PDSE, or UNIX member as if it were a sequential data set. The system treats a striped extended-format data set as if it were a single-volume data set.

Rule: You cannot concatenate VSAM data sets.

End-of-Data-Set (EODAD) Processing. When the change from one data set to another is made, label exits are taken as required; automatic volume switching is also performed for multiple volume data sets. When your program reads past the end of a data set, control passes to your end-of-data-set (EODAD) routine only if the last data set in the concatenation has been processed.

Consecutive Data Sets on a Tape Volume. To save time when processing two consecutive sequential data sets on a single tape volume, specify LEAVE in your OPEN macro, or DISP=(OLD,PASS) in the DD statement, even if you otherwise would code DISP=(OLD,KEEP).