Buffering options

Database buffers are defined areas in virtual storage. When an application program processes a segment in the database, the entire block or CI containing the segment is read from the database into a buffer. The application program processes the segment while it is in the buffer.

If the processing involves modifying any segments in the buffer, the contents of the buffer must eventually be written back to the database so the database is current.

You need to choose the size and number of buffers that give you the maximum performance benefit. If your database uses OSAM, you might also decide to use OSAM sequential buffering. The subtopics in this topic can help you with these decisions.

Multiple buffers in virtual storage
You can specify both the number of buffers needed in virtual storage and their size. You can specify multiple buffers with different sizes.
Subpool buffer use chain
In the subpool, buffers are chained together in the order in which they have been used. This organization is called a use chain.
The buffer handler
When a buffer is needed, an internal component called the buffer handler selects the buffer at the bottom of the use chain, because buffers that are least recently used are less likely to contain data an application program needs to use again.
Background write option
If you use VSAM, you can reduce or eliminate wait time by using the background write option.
Shared resource pools
You can define multiple VSAM local shared resource pools. Multiple local shared resource pools allow you to specify multiple VSAM subpools of the same size.
Using separate subpools
If you have many VSAM data sets with similar or equal control interval sizes, you might get a performance advantage by replacing a single large subpool with separate subpools of identically sized buffers. Creating separate subpools of the same size for VSAM data sets offers benefits similar to OSAM multiple subpool support.
Hiperspace buffering
Multiple VSAM local shared resource pools enhance the benefits provided by Hiperspace™ buffering.
Buffer size
Pick buffer sizes that are equal to or larger than the size of the CIs and blocks that are read into the buffer.
Number of buffers
Pick an appropriate number of buffers of each size so buffers are available for use when they are needed, an optimum amount of data is kept in virtual storage during application program processing, and application program wait time is minimized.
VSAM buffer sizes
The buffer sizes (in bytes) that you can choose when using VSAM as the access method are.
OSAM buffer sizes
For OSAM data sets, choose a buffer size that is the same as a valid block size so that buffer space is not wasted. Valid block sizes for OSAM data sets are any size from 18 to 32768 bytes.
Specifying buffers
Specify the number of buffers and their size when the system is initialized.

Parent topic: Designing full-function databases