General buffer pool considerations

Small tuned pools in IMS tend to minimize paging. In addition, because paging is not controlled by the IMS system, it can escalate dramatically in a storage-constrained environment.

In contrast, IMS buffer pool I/O is controlled and can be estimated and measured against those estimates. Finally, using a page fault and page I/O operation is generally more costly in processor terms than using a BSAM or IMS OSAM I/O.

Therefore, optimization of IMS in this area requires adjusting pool sizes while keeping in mind the costs in terms of paging and path length of scanning a pool.

Page fixing the IMS buffer pools

Information for initial optimization of IMS buffer pools is explained in IMS buffer pool tuning. Apply these guidelines to your system to achieve a minimum value for the total I/O operations (pool I/O and paging I/O). This means that you must consider a trade-off between I/O and storage across all pools, as well as within a single pool. For example, halving the maximum size of a large PSB pool might result in a very small increase in I/Os, but giving that extra storage to the message queue or format buffer pools might result in reducing even more I/O saving.

The way you tune your pools depends upon your real storage environment. In general, increasing maximum pool sizes increases paging; reducing pool sizes reduces paging of those areas.

If the IMS pools are subject to paging and IMS is sharing the processor usage, you can consider page fixing the significant buffers necessary for message and database access. This can be useful in a non-dedicated IMS system with very low-transaction rate, where IMS might be paged out unless the pools are page fixed. In heavily loaded IMS systems, response times might benefit from page fixing the data communication pools, because they hold data over the longest periods.

The principal IMS pools that can be page fixed are:

  • Message queue pool (QBUF)
  • DMB pool (DLDP)
  • PSB pool (DLMP)
  • Message format buffer pool (MFBP)
  • Database subpools

If the DL/I address space option is used, two PSB pools exist: DLMP in the z/OS® common area and DPSB in the DL/I address space.

Tell IMS to page fix all these areas, except the database subpools, by naming the resource in control statements that are included in the DFSFIXxx member of IMS.PROCLIB. The corresponding names used in these control statements are shown in parentheses in the above list.

To page fix the database subpools, you use control statements in the DFSVSMxx member of IMS.PROCLIB. The IOBF statement controls OSAM subpools. VSAM pool page fixing is specified with the VSAMFIX keyword of the OPTIONS control statement.

Usually, I/O for a pool impacts performance less than page faulting through the pool. If storage is a constraint, using a smaller fixed pool is preferable to using a larger pool that is subject to paging. However, page fixing is unlikely to improve performance, and might lead to higher overall paging rates, if your system is dedicated to IMS. This is because page fixing leaves a smaller available page pool for the rest of the system to use. Unless some other action is taken to reduce the storage requirements, higher rates inevitably result.