Data Space and Memory Caching in IDMS

Products

• IDMS
• IDMS - Database

Issue/Introduction

How does IDMS Data spaces/Memory Caching?

Environment

Release: All supported releases.

Resolution

As of IDMS release 16.0; the MEMORY CACHE clause replaces DATA SPACE clause so that IDMS Release 16.0 automatically uses Z-storage instead of Data spaces when executing in a z/OS 1.2 or later operating system.

See File Cache in Memory.

Decide what database files should use Memory Cache (those with high I/Os) and then change the DMCL definition for each file to specify MEMORY CACHE YES.

Attributes can also be changed dynamically with DCMT VARY DMCL to change DMCL-wide MEMORY CACHE options or DCMT VARY FILE to change the MEMORY CACHE for a specific file.

Where does the actual space come from?

The storage (z-storage) comes from storage above the 64-bit address line. If you don't have 64-bit memory available at all then we will try to allocate a Data space. If that fails the file doesn't get cached.

Do any of the IDMS storage pools need to be increased?

No, since the storage is not obtained from the IDMS storage pools.

Are there any recommended changes required for the IDMS startup region parameter?

You need to verify that you have REGION=0M (which will allow unlimited MEMLIMIT). Release Summary 16.0 6.2.2 Exploiting File Cache in Memory documents three ways to implement MEMLIMIT.

If the MEMLIMIT value is set too low, a 'DC016100 64-BIT STORAGE OBTAIN Failed' message is received.

For example if you have two files that are about 2.8G; then the MEMLIMIT should be set at 3G (MEMLIMIT=3G).