Using z/OS LFAREA with Datacom index buffers?

Products

• Datacom
• Datacom/DB
• Datacom/AD

Issue/Introduction

When to use z/OS Large Frame Pages with Datacom Index buffers  (IXX/DXX) and how to calculate the needed MUF LFAREAsize?

 

Environment

Release : 15.0 and higher

 

Resolution

Exploiting Large Frame Pages for application performance enhancement is expected as there is reduction in Dynamic Address Translation time (DAT) simply

because there is one Lookaside Buffer Entry in LBT per page so instead of having 1 entry per 4k page (normal page size), one would have 1 entry per 1M or 2G page thus reducing the DAT activity.

 

If you want all your IXX and DXX buffers covered by Large Frames Pages then add the number of DXX buffers to the number of IXX buffers and multiple by the index size to calculate the needed MUF LFAREAsize.

Remember though, the Datacom Multi-User facility (MUF) will get only what the Operating  System has available  at that time.

MUF allocates all SYSPOOL index buffers prior to its enabling and keeps this storage for the life of the MUF. 

Therefore, you should carefully consider the effects of the Large Page Frame Area size specification on other jobs in the LPAR that might also want Large Page Frame support.

  

The Datacom® startup specification of LFAREAsize is the maximum amount of the z/OS LFAREA that Datacom® uses. 

These large pages are first used to back the virtual storage allocated by the SYSPOOL IXX buffers.

 If this does not exhaust the LFAREA specification, the remaining area is used to back the DXX buffers.

 If there is not enough LFAREA available to support the MUF startup specification, the remaining buffers are backed by normal 4k page frames. 

The message DB00283I INSUFFICIENT LARGE PAGE FRAMES, MULTI-USER USED nM is printed for this case, where n is replaced with the amount of LFAREA MUF successfully acquired.

Additional Information

Refer to the Z/OS documentation for more details on the LFAREA parameter here