When running more than one DB2 z/OS subsystem on a single LPAR, there is a danger a "runaway" (e.g. storage leak) DB2 subsystem is taking out the LPAR. As this could cause a domino effect of outages caused by that single DB2 subsystem, REALSTORAGE_MAX subsystem parameter was introduced. This parameter was hidden ZPARM SPRMRSMX in DB2 V9 and now with PM24723/PK18354 becomes opaque. With DB2 10 you now have REALSTORAGE_MANAGEMENT and REALSTORAGE_MAX DB2 z/OS system parameters.
As mentioned above, it is recommend to set REALSTORAGE_MAX if more than one DB2 subsystems resides on a single LPAR. The best practices as of today are to set this parameter 1.5 to 2 x from the normal DB2 subsystem storage usage. If REALSTORAGE_MAX (DB2 10) or SPRMRSMX in DB2 9 is set to low, the DB2 subsystem might gets terminated before a real storage problem exists. If you set it to high, the LPAR might be gone before the parameter is being considered.
With REALSTORAGE_MANAGEMENT you have three options:
- ON - DB2 discards unused frames)
- OFF - DB2 should never discard unused frames)
- AUTO - DB2 detects paging impact and will try to reduce the frames counts avoiding system paging.
In addition new DB2 10 messages are going along with the actions
- DSNV516I... STORAGE CONTRACTION MODE
- DSNS002I... THE VALUE SPECIFIED FOR SUBSYSTEM PARAMETER REALSTORAGE_MAX IS EQUAL OR GREATER THAN AVAILABLE REAL STORAGE,
- DSNS003I ... WARNING THRESHOLD FOR REAL STORAGE HAS BEEN DETECTED )