OPTBLKSZ—set optimum block size (used with StopX37/II)
The block size of a data set has a direct effect on the storage efficiency of DASD devices. Generally, the larger block size fits more records on a track, which achieves better utilization of disk space and reduces the number of I/Os that are required to read the data set. Very small data sets might not benefit significantly from the denser record storage. However, because the minimum allocation by z/OS is a single track, allocating larger block sizes is generally more effective. The only disadvantage is that larger block sizes require larger buffers in memory. Table 1 shows the effect of different block sizes on DASD capacity:
Block size and DASD capacity
Record density, 3380 DASD, 80-byte records | |||
|---|---|---|---|
Block size | Records per track | Records per cylinder | Percentage of space used 1 |
4240 | 530 | 7950 | 90.0 |
6320 | 553 | 8295 | 93.7 |
23440 | 586 | 8790 | 98.9 |
- Source: IBM 3380 Direct Access Storage: Reference Summary
Despite the flexibility offered by this function in allowing various block sizes to be assigned, BMC Software recommends that when new applications are created, no hardcoded values of block size should be specified in either JCL or program code. Instead, use either an SMS Dataclass or the function OPTBLKSZ to assign a block size of 0 when creating a data set. SMS will then pick a block size that is appropriate for the device that is being used. Should the data set move to a different device type, SMS can reassign a different block size that is optimal for the new device without the need for JCL or program modifications.
For more information, see: