TSS Data Set Update Considerations
You can use any of the following macros or parameters to establish shared access to a TSS data set:
- MVS macros ENQ and DEQ
- OLD or SHR parameters of the ETATSS TSO command
- OLD or SHR parameters of the ETATSS JCL command statement
If you attempt to access an existing data set, the disposition of a TSS data set depends entirely upon the disposition that was assigned to the data set when it was allocated. When you allocate a TSS data set, the default disposition is SHR.
If you attempt to update an existing TSS data set, the OLD and SHR keywords on the ETATSS command or command statement and the Disposition field on the TSS panels of the ETA online interface are ignored because the data set has been allocated and a disposition has been specified.
If you allocate a TSS data set with a disposition of OLD, MVS assigns the data set exclusively to one user. In this case, updates from this one user are always allowed and TSS does not perform special enqueues. The update mode keywords EXCL, SHAR, and NONE are ignored.
If you use a disposition of SHR, TSS responds to the update mode keywords EXCL, SHAR, and NONE. To protect the data set from simultaneous updates by other users, TSS issues a systems-level enqueue on the data set.