Monitoring and Tuning Database Performance › Reducing I/O › By Caching Files in Memory

By Caching Files in Memory

If a database file is cached in memory, the DBMS looks in the cache before reading a database page from disk. If the page resides in the cache, the retrieval I/O is eliminated. If the page must be read from the disk, it is saved in the cache to satisfy future retrieval operations. Database files with a high number of I/Os are good candidates for caching in memory.

There are two basic types of file caching: shared cache which uses coupling facility services to enable a single cache to be shared by multiple central versions and memory cache which is accessible only by a single central version. The remainder of this discussion focuses on memory cache. For information about shared cache, see the CA IDMS System Operations Guide.

Note: Memory caching is available only for non-native VSAM files.

To enable the use of memory cache, take the following steps:

• Decide which files to cache by using standard performance-monitoring tools to determine the database files with the most I/O. For example, you can use the DCMT DISPLAY STATISTICS FILES to get a list of all files and their associated I/O counts or look at gathered operating system statistics. Choose files with the highest retrieval counts.
• Change the DMCL definition to specify MEMORY CACHE YES for each file to be cached. For details, see DMCL Statements. Alternatively, use the DCMT VARY FILE command to dynamically initiate the use of memory cache for one or more files. For more information, see the CA IDMS System Tasks and Operator Commands Guide.
• Compute the total amount of storage that is needed to cache the selected files. To do this, for each file, multiply the number of blocks in the file by the file's block size and total the results. The resulting value is the amount of storage needed. Ensure that sufficient storage of the required type is available to all jobs that use the altered DMCL.

If the operating system supports 64-bit storage, the cache is allocated in 64-bit storage if sufficient memory is available. If no or not enough 64-bit storage is available to hold the entire file, the file will not be cached in memory. For details, see 7.13, "DMCL Statements".

Note: For more information about operating-specific considerations in using memory cache and 64-bit storage, see the CA IDMS System Operations Guide.