Restart light in data sharing

The LIGHT(YES), LIGHT(NOINDOUBTS), and LIGHT(CASTOUT) clauses of the -START DB2 command restart a member in light mode.

Restart light mode does the following:
  • Minimizes the overall storage required to restart the member.
  • Removes retained locks as soon as possible, except for the following:
    • Locks that are held by postponed-abort units of recovery, if the LBACKOUT subsystem parameter is set to LIGHT or LIGHTAUTO.
    • IX and SIX mode page set P-locks, if LIGHT(YES) or LIGHT(NOINDOUBTS) is specified. These locks do not block access by other members; however, they do block drainers, such as utilities.
  • Terminates the member normally after forward and backward recovery is complete. No new work is accepted.

Start of changeWhen starting data sharing members with light restart, it is best to start one data sharing member at a time.End of change

Restart light mode is not recommended for a restart in place. It is intended only for a cross-system restart of a system that does not have adequate capacity to sustain the Db2 and IRLM in the event of a failed z/OS® system.

Tip: Restart any Db2 data sharing member that goes through restart light again with a regular start. Otherwise, certain lock avoidance optimizations can remain disabled for all data sharing members. For more information about the optimizations that might become disabled, see Global commit LRSN enhancement in Db2 12 for z/OS Performance Topics (IBM Redbooks).

Indoubt units of recovery

When you restart a member by using the LIGHT(NOINDOUBTS) clause of the -START DB2 command, the member terminates without waiting for resolution of indoubt units of recovery (URs).

When you restart a member by using the LIGHT(YES) or LIGHT(CASTOUT) clause and indoubt URs exist, Db2 issues message DSNR052I at the end of restart. The member continues to run in light mode to allow the indoubt URs to be recovered.

When Db2 for z/OS operates in restart light mode, it resolves indoubt URs in environments in which Db2 for z/OS is a participant relative to a local or a remote coordinator that makes the decision to commit or abort an indoubt UR. Db2 for z/OS can also be a coordinator relative to local or remote participants. As a coordinator, Db2 provides commit and abort decisions to remote participants as long as Db2 and DDF remain started while resolving participant-related indoubt URs. In either of those environments, Db2 automatically terminates after all indoubt URs have been resolved.

XA recovery processing for a remote XA Transaction Manager (XA TM) requires that the XA TM connect to the SQL port (tcpport) of the data sharing group to retrieve a list of transaction identifiers (XIDs) that are indoubt. Because the SQL port is unavailable for the restart light member, another member of the data sharing group must be available and accessible through a group DVIPA. After the available member is contacted, that member returns the member DVIPA and resynchronization port (resport) for any indoubt XIDs that are owned by the restart light member. The XA TM resolves work at the restart light member because its resynchronization port is available.

Indoubt URs can be resolved in two ways: automatically through resynchronization processing or manually.
  • Automatically: Ensure that the appropriate commit coordinators (IMS or CICS® subsystems that have indoubt URs on the restart-light member) are started so that they can resynchronize with the member to resolve the indoubt URs.
  • Manually: Use the RECOVER INDOUBT command to manually resolve the indoubt URs.
Use the DISPLAY THREAD command to monitor the progress of indoubt UR resolution.

After the last indoubt UR is resolved, the member terminates. Alternatively, if you want to stop the member before all indoubt URs are resolved, you can issue the -STOP DB2 command. Db2 issues message DSNR046I to indicate that incomplete URs still exist.

Db2 commands and restart-light mode

The following commands are not allowed when a member is started in light mode:
  • DISPLAY DATABASE, START DATABASE, STOP DATABASE
  • DISPLAY RLIMIT, START RLIMIT, STOP RLIMIT
  • SET SYSPARM
  • START DDF, STOP DDF