Using 64-bit memory to handle spill files

The purpose of spill files is to buffer log records when there is a requirement to maintain the serialization of log records and when those records cannot be immediately processed.

Fast Path spill files buffer the log input records when parallel log readers are assigned.

Spill files can also be created for CAGRP sort tasks. In single address space mode, each sort task creates a unique spill file. For multiple address space mode, spill files will be used when the number of CAGRP sorts to be processed exceeds the parallel sort count.

Prior to this release of IMS HP Change Accumulation Utility, spill files were always written to DASD or tape. In this release, IMS HP Change Accumulation Utility can take advantage of 64-bit addressing to store and retrieve spill file data in memory, thereby reducing the I/O activity to external storage devices.

Using 64-bit memory can reduce execute channel program (EXCP) count to improve performance. If the 64-bit memory is set large enough, all the spill records will go to 64-bit memory. If the 64-bit memory for the address space becomes full, the remaining spill records will be written to spill files on DASD or tape. As spill records are retrieved and processed, storage will be released to be reused for subsequent spill files.

Two mutually exclusive application methods are available for accessing spill records in 64-bit memory above the bar:

• The IMS HP Change Accumulation Utility MEMLIMIT command specifies the amount of storage above the bar that IMS HP Change Accumulation Utility is allowed to access.

  The IMS HP Change Accumulation Utility MEMLIMIT command is an option when IMS HP Change Accumulation Utility runs in either multiple address space mode or single address space mode.

• The IMS HP Change Accumulation Utility cache manager utilizes an external file (HPCCACHE DD) and control statements to define the 64-bit limits.

  The cache manager is the preferred method only when IMS HP Change Accumulation Utility runs in single address space mode.

Each method defines an application limit to the amount of storage that can be used above the bar. Both methods are equally restricted such that the application-defined limit is always subject to the system-applied limits.

In addition to IMS HP Change Accumulation Utility, the DFSORT task(s) can also specify the use of storage above the bar for sort work space. The 64-bit storage defined by IMS HP Change Accumulation Utility does not have to encompass all of the available storage. For example, the JOB statement parameter MEMLIMIT=10G will make 10 gigabytes above the bar available to the address space. If the cache manager specifies SPACE=2G, then 8 GB of storage above the bar will be available to DFSORT.