Utilities › TMSPTRS Utility—Maintain Multivolume, Multi-Data Set Chains › TMC Chaining Structures

TMC Chaining Structures

Understanding the various structures found in the TMC is helpful in correcting any errors reported. Multivolume chains are carried in five fields of each TMC Volume record associated with the data set, VOLSEQ, VOLSER, 1STVOL, NEXTVOL and PREVVOL, using forward and backward pointers. All Volume records in a volume chain must contain the same value in the 1STVOL field.

Multi-data set chains are reflected in three fields of the TMC Volume record, 1STDSNB, LSTDSNB, and NUMDSNB, and in three fields of the DSNB record, VOLSER, PREV and NEXT. DSNB chains are constructed with forward and backward pointers. All DSNB records in a DSNB chain must contain the same value in the VOLSER field.

Multivolume/multi-data set (aggregate) chains use all chaining fields in TMC and DSNB records. DSNB chains are constructed with forward and backward pointers starting with the first volume that has the second file created on it. The DSNB total must be reflected in the last volume associated with the aggregate.

The free chain of DSNBs, that is, those DSNBs that are unused and available to track new secondary data sets, is maintained through a pointer field, NXAVAIL, contained in Control Record #2 (TMSCTL#2) of the TMC. In a newly formatted TMC, the DSNBs are consecutive off of Control Record #2. All unused DSNB records in the free chain should have bit DSNBACTV of the active indicator field (ACTIND) turned off. The last DSNB should never be used and must always contain HEXZEROS in the NEXT field.

Note: The PREV field in the DSNB is not used in the free chain.

As DSNBs are allocated, the NXAVAIL field in Control Record #2 is altered to reflect the next unused DSNB in the free chain. The USED field is incremented by one to reflect the number of DSNB records already used. The active indicator (ACTIND) is turned on in the DSNB.

During scratch processing, TMSCLEAN clears the DSNB active indicator (ACTIND) and returns the DSNB to the front of the free chain. The NEXT field in the DSNB record is updated to reflect the current NXAVAIL value in Control Record #2, the NXAVAIL field is updated with the DSNB number being returned to the free chain, and the USED field is decremented by one.

Note: After the DSNBs have been used and returned to the free chain, the DSNBs are no longer consecutive off of Control Record #2.