Control interval sizing
A control interval is an area on disk where VSAM stores records and creates distributed free space. A control interval is a unit of information that VSAM transfers between virtual and auxiliary storage.
Db2 page sets are defined as VSAM linear data sets. Db2 can define data sets with variable VSAM control intervals. One of the biggest benefits of variable VSAM control intervals is an improvement in query processing performance.
The VARY DS CONTROL INTERVAL parameter on installation panel DSNTIP7 allows you to control whether Db2-managed data sets have variable VSAM control intervals:
- A value of YES indicates that a Db2-managed data set is created with a VSAM control interval that corresponds to the size of the buffer pool that is used for the table space. This is the default value.
- A value of NO indicates that a Db2-managed data set is created with a fixed VSAM control interval of 4 KB, regardless of the size of the buffer pool that is used for the table space.
The following table shows the default and compatible control interval sizes for each table space page size. For example, a table space with pages that are 16 KB in size can have a VSAM control interval of 4 KB or 16 KB. Control interval sizing has no impact on indexes. Index pages are always 4 KB in size.
| Table space page size | Default control interval size | Compatible control interval sizes |
|---|---|---|
| 4 KB | 4 KB | 4 KB |
| 8 KB | 8 KB | 4 KB, 8 KB |
| 16 KB | 16 KB | 4 KB, 16 KB |
| 32 KB | 32 KB | 4 KB, 32 KB |