MANAGE REAL STORAGE field (REALSTORAGE_MANAGEMENT subsystem parameter)

The REALSTORAGE_MANAGEMENT subsystem parameter specifies whether Db2 should manage real storage consumption.

Acceptable values: AUTO, AUTO1, OFF, ON
Default: AUTO
Update: option 20 on panel DSNTIPB
DSNZPxxx: DSN6SPRM REALSTORAGE_MANAGEMENT
Important:

Before you apply the PTF for APAR PH47163, Db2 12 always uses the AUTO1 behavior when AUTO is specified. In most cases, it is best to use AUTO1 only if you encounter problems after applying this PTF. Also, in Db2 13 or later, this subsystem parameter is removed, and Db2 always manages real storage automatically.

AUTO
Start of changeDb2 checks the available real storage on the LPAR, and uses the following threshold calculation to determine the storage management behavior to use. 
(REALAVAIL > 1.2 * MAXSPACE)
REALAVAIL is the amount of currently available real storage on the LPAR, and MAXSPACE is a setting that restricts the virtual storage available to the z/OS® DUMPSRV address space.
Db2 switches behaviors based on the threshold as described in the following table. Db2 issues message DSNS006I when it switches behaviors.Start of change
If the amount of available real storage is... Db2 switches to behavior of the...
Greater than the threshold OFF setting, and Db2 does not discard unused real storage at thread deallocation or during commit.
Less than or equal to the threshold AUTO1 setting
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Start of changeAUTO1End of change
Start of changeStart of changeDb2 discards thread storage at thread deallocation, or every 120 commits if the thread is reused.

If the available real storage on the LPAR drops below the z/OS Available Frame Queue Low threshold, Db2 enables contraction mode.

End of changeEnd of change
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In contraction mode, Db2 discards every 30 commits. Db2 indicates when it enters contraction mode by issuing console messages DSNV516I and DSNS005I with reason LOW ON AVAILABLE FRAMES. When Db2 leaves contraction mode, it issues message DSNV517I.

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OFF
Db2 does not discard unused real storage frames, unless one of the following conditions is met:
  • The LPAR reaches an auxiliary critical state.
  • Start of changeThe total real and auxiliary storage reaches the limit specified by the value of the REALSTORAGE_MAX subsystem parameter.End of change
ON
Db2 always operates in contraction mode. Discarding the frames results in some CPU overhead, and this option is intended for systems in which the availability of real storage is limited. This value is most likely appropriate for LPARs that have many Db2 subsystems, such as a development LPAR.