Enabling z/TPFDF cache support for a file
To use z/TPFDF cache support to cache subfiles for a z/TPFDF file, you must ensure that z/TPFDF cache support is both active for the system and enabled for that file. You can enable z/TPFDF cache support for the file before or after you activate z/TPFDF cache support for the system, but caching does not start until z/TPFDF cache support is both active and enabled for that file.
About this task
To enable a z/TPFDF file to use z/TPFDF cache support, that file must be defined with a block limit, which determines the maximum number of blocks to be cached for each subfile in that z/TPFDF file. You can use the block limit to control whether to partially cache each subfile or cache each subfile as a whole. For example, if a subfile contains 15 blocks, and you specify the block limit as 10, the first 10 blocks of the subfile are cached, including the prime block and nine overflow blocks. If a subfile read request requires access beyond the first 10 blocks, any additional overflow blocks of the subfile are read from the DASD as needed. You can specify the LIMIT parameter on the ZUDFM CACHE command to define the block limit. The CACHE_PRTL_IOCNT and CACHE_PRTL_IOREQ counters, which are available in CDC data reported by runtime metrics collection, provide information about partially cached subfiles. Based on this information, you can tune your system settings to use z/TPFDF cache support more efficiently.
Procedure
- If you previously defined the block limit for the file by specifying the LIMIT parameter on the ZUDFM CACHE command, enter the ZUDFM CACHE command with the FILE and ENABLE parameters specified.
- If you previously did not define the block limit for the file by specifying the LIMIT parameter on the ZUDFM CACHE command, enter the ZUDFM CACHE command with the FILE, ENABLE, and LIMIT parameters specified.
Results
- If z/TPFDF cache support is active, the z/TPFDF product starts caching subfiles for the enabled file.
- If z/TPFDF cache support is inactive, the z/TPFDF product starts caching subfiles for the enabled file the next time z/TPFDF cache support is activated.