File Cache in Memory

With the introduction of 64-bit hardware and the operating systems which could exploit it, the theoretical amount of virtual storage available to an application increased to 16 exabytes. CA IDMS can exploit this high amount of storage by caching entire database files in memory.
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With the introduction of 64-bit hardware and the operating systems which could exploit it, the theoretical amount of virtual storage available to an application increased to 16 exabytes. CA IDMS can exploit this high amount of storage by caching entire database files in memory.
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Key Benefits 
  • Reduced number of I/Os
  • Increased throughput
  • Less CPU usage
For more information on 64-bit addressing, see the IBM manuals,
z/OS MVS Extended Addressability Guide
or
z/VSE Extended Addressability.
Terminology
The following terms are used in this discussion of file cache in memory:
  • The bar
    -- The bar marks the 2-gigabyte limit of 31-bit addressing. This is analogous to
    the line
    , which marks the 16-megabyte limit of 24-bit addressing.
  • Z-storage
    -- Virtual storage above the bar.
Exploiting File Cache in Memory
Database files with a high number of I/Os are good candidates for the file cache in memory feature. The DBA should use standard performance-monitoring tools to determine which database files these are. Once the decision is made as to which files will use this feature, the DBA should perform these steps:
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 sum all results. This sum is the total amount of Z-storage needed for file caching. Be aware that if the "Scratch in Storage" SYSGEN option is specified, it may also use Z-Storage.
  • Make sure that the jobs that use the modified DMCL have enough Z-storage (at least the amount computed above) at their disposal. The amount of Z-storage available to a job is limited by the MEMLIMIT parameter. For an explanation of MEMLIMIT, see the IBM manuals,
    z/OS MVS Extended Addressability Guide
    or
    z/VSE Extended Addressability
    .
    For z/OS you can set MEMLIMIT in the following ways:
    • Through an installation default. For more information, see the IBM manual,
      z/OS MVS Initialization and Tuning Reference
      .
    • In the JOB and EXEC statements. For more information, see the IBM manual,
      z/OS MVS JCL Reference
      .
    • Through an installation exit. For more information, see the IBM manual,
      z/OS MVS Installation Exits
      .
    For z/VSE you can set the MEMLIMIT through the SYSDEF MEMOBJ command. For more information on this command, refer to the IBM manual
    z/VSE System Control Statements
    .
  • Change the DMCL definition for each file to specify MEMORY CACHE YES.
    For more information on changing the DMCL definition, see the
    Database Administering section
    .It is also possible to dynamically change the MEMORY CACHE attribute for a file using the DCMT VARY FILE command. For more information on DCMT commands, see the
    System Tasks and Operator Reference section
    .
    On z/VSE, CV must run in a single-partition address space to use Z-storage.