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z/OS DFSMS Using Data Sets
Sometimes a record is larger than the control interval size used for a particular data set. In VSAM, you do not need to break apart or reformat such records, because you can specify spanned records when defining a data set. The SPANNED parameter permits a record to extend across or span control interval boundaries.
Spanned records might reduce the amount of DASD space required for a data set when data records vary significantly in length, or when the average record length is larger compared to the CI size. The following figures show the use of spanned records for more efficient use of space.
In Figure 1, each control interval is 10 240 bytes long.
Figure 1. Data set with nonspanned records
In Figure 1 control interval 1 contains a 2000-byte record. Control interval 2 contains a 10 000-byte record. Control interval 3 contains a 2000-byte record. All together, these three records use 30 720 bytes of storage.
Figure 2. Data set with spanned records
The control interval size is reduced to 4096 bytes. When the record to be stored is larger than the control interval size, the record is spanned between control intervals. In Figure 2, control interval 1 contains a 2000-byte record. Control intervals 2, 3, and 4 together contain one 10 000-byte record. Control interval 5 contains a 2000-byte record. By changing control interval size and permitting spanned records, you can store the three records in 20 480 bytes, reducing the amount of storage needed by 10 240 bytes.
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