LFAREA parameter

The IEASYSxx LFAREA parameter specifies the maximum amount of real storage that can be used to satisfy fixed 1 MB and 2 GB page requests. All fixed 1 MB and 2 GB pages are backed by contiguous 4 KB real storage frames. Specifying the LFAREA parameter for fixed 1 MB pages does not reserve an amount of real storage to be used exclusively for fixed 1 MB page requests. If the system becomes constrained by a lack of sufficient 4 KB frames to handle workload demand, it can use any available real storage specified by the LFAREA 1M parameter to back 4 KB page requests, enabling the system to react dynamically to changing system storage frame requirements.

If 1 MB large frames are required but none are currently available, the system attempts to defragment real storage to form 1 MB large frames. However, frequent defragment attempts can indicate a system configuration and tuning issue. To resolve this issue, you can either decrease the size of the LFAREA, adjust the workload to reduce the demand for 4 KB frames, or add more real storage.

Because the IEASYSxx LFAREA parameter requires an IPL in order to change the LFAREA value, the following considerations apply:
  • If the value specified for LFAREA is too small, available 1 MB and 2 GB pages might not exist for applications that could benefit from large page usage.
  • The LFAREA request can be specified with target and minimum values for 2 GB and 1 MB pages. Additionally, a NOPROMPT option allows the IPL to continue with no LFAREA with no operator intervention even if the minimum value is not met.
  • Determine the total number of 1 MB and 2 GB pages that your applications require, and consider using this number as a starting point for your LFAREA target values. Start of changeAlso add a small amount of 1 MB pages to your starting point for use by the system. Use your existing performance monitor to determine how much 1 MB pages are used by your system.End of change
  • Use output from the MODIFY AXR,IAXDMEM system command as an estimate for the maximum number of 1 MB pages used on behalf of fixed 1 MB page requests. Use this estimate to determine if your LFAREA value is too small; refer to the IAR049I message in z/OS MVS System Messages, Vol 6 (GOS-IEA) for additional details.
Refer to the following documentation for additional details about LFAREA:
  • For specific details on specifying the IEASYSxx LFAREA parameter, see LFAREA parameter in IEASYSxx in z/OS MVS Initialization and Tuning Reference.
  • For information on calculating the LFAREA value based on DB2® requests, see IBM® DB2 10 for z/OS® Managing Performance.
  • For information on calculating the LFAREA value based on JAVA heaps, see IBM SDK for z/OS, Java™ Technology Edition.