ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.41 VSAM: Data-in-virtual (DIV)
DIV
(Address space)
(Dataspace)
or
(Hiperspace)
Figure 4-51 Data-in-virtual (DIV)
Data-in-virtual (DIV)
You can access a linear data set using these techniques:
► VSAM
► DIV, if the control interval size is 4096 bytes. The data-in-virtual (DIV) macro provides
access to VSAM linear data sets.
► Window services, if the control interval size is 4096 bytes.
Data-in-virtual (DIV) is an optional and unique buffering technique used for LDS data sets.
Application programs can use DIV to map a data set (or a portion of a data set) into an
address space, a data space, or a hiperspace. An LDS cluster is sometimes referred to as a
DIV object. After setting the environment, the LDS cluster looks to the application as a table
in virtual storage with no need of issuing I/O requests.
Data is read into main storage by the paging algorithms only when that block is actually
referenced. During RSM page-steal processing, only changed pages are written to the cluster
in DASD. Unchanged pages are discarded since they can be retrieved again from the
permanent data set.
DIV is designed to improve the performance of applications that process large files
non-sequentially and process them with significant locality of reference. It reduces the
number of I/O operations that are traditionally associated with data retrieval. Likely candidates
are large arrays or table files.
174 ABCs of z/OS System Programming Volume 3
Enable users to:
Map data set to virtual storage
Access data by exploiting paging
algorithms