Wednesday, October 10, 2007

RECOVER Improvements [DB2 9 for z/OS]

Our examination of the new utility functionality in DB2 9 for z/OS continues today as we uncover what’s new with the IBM RECOVER utility.

Point-In-Time Consistency

The most significant enhancement centers on recovering to a point-in-time (PIT) with consistency. As any DBA who has ever been charged with recovering an operational database to a prior PIT knows, it can be challenging to accomplish. Of course, if you have a usable QUIESCE point then point in time recovery is easy. But that means you’d have to have planned ahead of time and taken a QUIESCE, which is not always possible with heavy 24x7 workloads.

Well, DB2 V9 offers some relief because the IBM RECOVER utility is enhanced to automatically identify uncommitted work at the PIT to which you are recovering. After detecting the uncommitted work DB2 will rollback any changes made to the database objects that are being recovered. This means that anything that is being recovered will be in a transactionally consistent state. Of course, you still have to make sure that all of the appropriate objects are participating in the recovery.

This works with for PIT recoveries executed with either the TOLOGPOINT or TORBA keywords only. To accomplish this two new phases are added after the LOGAPPLY phase: LOGSCR and LOGUNDO.

The LOGSCR phase runs for each DB2 member with an active unit of recovery (UR). If no UR was active then this phase will be skipped. During the LOGCSR phase, all active and incomplete URs are identified. This includes URs that were:

If any URs were found, the LOGUNDO phase runs. This is where RECOVER backs out
the changes made on recovered objects by active URs. In a data sharing environment, this
will be done one member at a time.

Additional Information on the Progress of Your Recovery

Another improvement made to the IBM RECOVER utility in V9 is the ability to gather more information about the progress of the recovery. Any DBA who has been tasked with sitting around and monitoring recoveries what the DISPLAY command to show accurate, up-to-date, and appropriate information about the status of the recovery.

Well, DB2 V9 enhances the -DISPLAY UTILITY command to provide additional information during the LOGAPPLY phase. DISPLAY now will show the range of the log that needs to be applied, for the database objects that are being recovered. It will also show the progress up to the last commit, as well as the elapsed time since the start of the log apply phase of the recovery.

This information will assist the DBA in determining how long it will take for the recovery job to successfully complete.

Using an Earlier Image Copy

One final enhancement made to the RECOVER utility in V9 gives the DBA control to get DB2 to recover using an earlier image copy. This is accomplished by specifying a point in the log before which DB2 must select an image copy for the recovery.

This technique can be particularly helpful when the most recent image copy is damaged or somehow unusable. You can use an earlier copy by providing an RBA/LRSN value to the new RESTOREBEFORE parameter that is at a point on the log prior to when the unusable image copy was taken.

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