An indexed sequential data set must be reorganized occasionally
for two reasons: The overflow area eventually fill. Additions increase
the time required to locate records directly. The frequency of reorganization
depends on the activity of the data set and on your timing and storage
requirements. There are two ways to reorganize the data set:
- In two passes by writing it sequentially into another area of direct access storage or magnetic tape and re-creating it in the original area.
- In one pass by writing it directly into another area of direct access storage. In this case, the area occupied by the original data set cannot be used by the reorganized data set.
The operating system maintains statistics that are pertinent to reorganization. The statistics, written on the direct access volume and available in the DCB for checking, include the number of cylinder overflow areas, the number of unused tracks in the independent overflow area, and the number of references to overflow records other than the first. They appear in the RORG1, RORG2, and RORG3 fields of the DCB.
When creating or updating the data set, if you want to be able to flag records for deletion during updating, set the delete code (the first byte of a fixed-length record or the fifth byte of a variable-length record) to X'FF'. Figure 1 describes the process for deleting indexed data set records, and how a flagged record will not be rewritten in the overflow area after it has been forced off its prime track (unless it has the highest key on that cylinder) during a subsequent update.
Figure 1. Deleting Records from an Indexed Sequential Data Set
Similarly, when you process sequentially, flagged records are not retrieved for processing. During direct processing, flagged records are retrieved the same as any other records, and you should check them for the delete code.
A WRITE KN instruction for a data set containing variable-length records removes all the deleted records from that prime data track. Also, to use the delete option, RKP must be greater than 0 for fixed-length records and greater than 4 for variable-length records.