Implications of changing from Daylight Savings to Standard Time for (1) the DBRC user with databases registered, (2) the non-DBRC Pre-IMS V6 user (3) the DBRC user with no data bases registered and the non-DBRC IMS V6 or V7 User
IMS Implication of Changing from Daylight Savings to Standard Time
Each autumn most of the United States changes from Daylight Savings time to Standard time. At some time the MVS local times will be set back one hour. Officially, this will occur at 2:00 AM. Similar time changes are made in many other countries. This changing of the MVS local time could have serious implications for the user who is running IMS subsystems during this period, since data base recovery is sensitive to the log record time stamps.
The words "IMS subsystem" will appear many times in the following text. IMS subsystems include IMS online (DC and DBCTL) systems, IMS DLI/DBB batch jobs, CICS - DL/I systems, the IMS data base utilities (image copy, change accum, recovery, unload, reload, prefix update, etc.), and the logging utilities (log recovery and archive).
If all IMS subsystems are IMS V6 or V7 and a CHANGE RECON NOCOEX operation has been performed (the CHANGE.RECON NOCOEX command tells DBRC that this RECON will never be accessed by a pre-IMS V6 subsystem), then you do not need to do anything when the local time is changed (i.e., subsystems do not need to be stopped) and you can ignore the rest of this FLASH.
DBRC User - Databases Registered
The DBRC user who has databases registered must perform the following steps when changing the MVS local time, if any pre-IMS V6 subsystems are (or could be) sharing the RECONs.
- Terminate all IMS subsystems within the IMS Sysplex.
- Change (move backward) all of the MVS local times
- If using a RECON that has been upgraded to IMS V6 or V7 format, issue a CHANGE.RECON THT command, using an IMS V6 or V7 RESLIB.
- Wait an hour. IMS V6 and V7 subsystems will enforce this wait. Prior versions of IMS will not.
- Restart the IMS subsystems.
For further information regarding the impact of issuing (or not issuing) the CHANGE.RECON THT command for V6 or V7 RECONs, see the text for IMS APAR PQ03038.
Non-DBRC Pre-IMS V6 User
A number of options exist for the pre-IMS V6 user who is not using DBRC (not DBCTL or DB/DC - i.e., no RECONs.) These include:
- Don't run any IMS subsystems during the "duplicate" time period (the one hour period between the original 2:00 AM and the new 2:00 AM). This option, by far and away, is the easiest option to implement.
- Bring all IMS subsystems down immediately prior to resetting the local times, reset the local times, backup all data bases updated in the preceding hour, and restart the subsystems.
- If no time is available for backing up the data bases between the time the subsystems were brought down, the local times reset, and the subsystems restarted, then the user must remember which logs were created under daylight savings time and which logs were created under standard time. Any data base recoveries that use a backup created under daylight savings time will have to be performed in two steps. The first step would recover the data base up to the time change and would use the backup (image copy) and logs created prior to the time change. Once that had been done, then a second recovery will have to be performed using the logs created after the time change. It should be noted that if change accumulation input is used for the recovery, two change accumulation operations must be performed, one using logs created prior to the time change and the other using logs created after the time change. In addition, the second change accumulation run must be run with the old change accumulation input dummied out.
All IMS subsystems must be terminated prior to the time change. IMS subsystems generally use the hardware clock (STCK instruction) rather than the MVS TIME macro to obtain the time. At subsystem initialization time, IMS computes an adjustment factor to convert the STCK value to a local date and time. This adjustment factor could be recomputed at midnight under certain, unpredictable circumstances. If a user were to reset the MVS local time while an IMS subsystem was running, the subsystem would not recognize the time change immediately. However, IMS subsystems started after the time change would use the new time. Thus, the new subsystems would be an hour behind the old subsystems. In addition, if the IMS subsystems that were running prior to the time change continued execution past midnight (old time), then they could reset their internal clocks back an hour and cause negative time breaks in the log.
DBRC User - No Data Bases Registered
The user who is only using DBRC to manage the online logs (i.e., no data bases registered) has the same options as the non-DBRC pre-IMS V6 user (prior section) with the following restriction if options 2 or 3 are selected.
When the online systems are brought down, the user must switch to a 'clean' set of RECONs (i.e., no PRIOLDS, PRISLDS, or PRILOG records for the subsystem), issue a DBRC CHANGE.RECON THT command following the MVS local time changes if using a V6 or V7 RECON, and then bring the online systems back up with a cold start.
Non-DBRC IMS V6 or V7 User
An IMS V6 or V7 user who is not using DBRC (not using DBCTL or IMS TM - i.e., no RECONs) does not have to do anything. IMS V6 and V7 use UTC (GMT) time when time stamping database change records, image copy headers, etc. As a result, changing local time has no impact on recovery.
Author: Bill Keene
Updated by: Rich Lewis