IMS Support for VSAM Linear database Buffer pools 64 bit addressable

Support for VSAM LINEAR datasets for IMS databases was delivered for the enhancement for databases dataset encryption in 2020. The buffer pools for VSAM LINEAR datasets are still treated like OSAM buffer pools with 31 bit addressability. Anthem has a very constrained IMSPlex with 4 production IMS regions in the plex. 2 for transactions and 2 for BMPs. Currently we can only allow 5 MB of free virtual storage 31 bit addressable without having to reduce the buffer pools and decrease performance. This is a large HALDB environment with most transaction and BMP PSBs referencing the larger HALDBs. The IMS DB application teams have been in progress of converting DLI batch jobs to BMPs but the progress is slow due to lack of virtual storage available for larger buffer pools and OSAM SB so performance will likely suffer.

Some statistics on the virtual storage break down is below:

Extended private region is about 1380 MB

The IMS DLI region allocates 1216 MB for user subpool storage requirements of which 862 MB is for OSAM and VSAM buffer pools. The region also allocates about 93 MB for ELSQA storage leaving about 70 MB available or slightly more than 5% of extended Private.

The 862 MB of buffer pool storage is about 62% of the extended Private region

The 862 MB of buffer pool storage is allocated as 91% OSAM and 9% VSAM

Our buffer pool hit ratio is about 98.6% suffering about 19,000 read I/Os per second

Conversion of the large OSAM HALDBs to VSAM LINEAR would be worth while if the buffer pools are allocated in 64 bit addressable storage.