Buffer Management › Managing Buffers Dynamically

Managing Buffers Dynamically

Changing Buffer Characteristics

Once a database is in operation, you can vary the characteristics of buffers dynamically by issuing the DCMT VARY BUFFER statement.

By making a temporary change to a buffer setting online, you can evaluate the potential impact this change might have on overall system performance. This allows you to identify the optimal settings for your buffers. When you have identified the optimal settings, you can make permanent changes to the buffer definitions by using the ALTER BUFFER DDL statement.

Types of Changes

The following buffer characteristics can be changed using DCMT commands:

• The number of pages in the buffer pool
• The number of pages to be acquired in each storage request (this value defaults to the initial number of pages in the buffer pool)
• The maximum number of pages in the buffer pool
• The storage acquisition mode (OPSYS or IDMS)
• Whether the chained read facility is activated and the number of pages that must be in the buffer to invoke chained reads as described under 18.4, “Using Chained Reads”
• Whether a file is cached using a DCMT VARY FILE/AREA/SEGMENT command

If the number of pages in the buffer pool is changed to any value between the initial and maximum number of pages, the change is effective immediately. Changing the number of pages in the buffer pool beyond this range or changing other buffer characteristics takes effect only after the buffer is closed and re-opened. The buffer can be closed using a DCMT VARY BUFFER command and it will be re-opened automatically when the next read occurs for a file associated with the buffer.

Varying a DMCL

The following buffer changes can be made dynamically by varying a new copy of the DMCL:

• The page size of a buffer can be changed
• New buffers can be added to the system
• Existing buffers can be removed from the system
• Files can be associated with a different buffer

Other characteristics, such as the number of pages in the buffer or the storage acquisition mode, are not affected by varying a new copy of the DMCL. To dynamically make such changes, use the DCMT VARY BUFFER command.