Aritcle-ARCHIVE LOG COMMAND BASICS

Archive Log Mode Quiesce basics.

This article aims to explain in simple terms how the Archive Log Mode quiesce commad works..

To review the implication of MODE QUIESCE let us look at the following situation.

CASE 1

Let us assume that there are two active transactions (Unit of Recoveries) A and B Running in our system while we issue the ARCHIVE LOG command with out MODE QUIESCE. Let us further assume that UOR A and UOR B are both actively changing the data and will not commit till some future times T1 and T2 respectively.

In this scenario, if we issue an ARCHIVE LOG command at time T0, then the following actions are taken.

A System wide checkpoint is taken by DB2 and recorded in the BSDS and the LOG
The log offload task is initiated and the control returned to the user or the job that issued the command

It should be evident by now that the log we just archived does not contain all the relevant records that pertain to UOW A and UOW B. If this log is used at the DR site, then we will end up with 2 in-doubt Unit of Works. This is because DB2 does not know if these two transactions Committed or Rolled back or how many records they changed before doing so.

Case 2

Let us assume that there are two active transactions (Unit of Recoveries) A and B Running in our system while we issue the ARCHIVE LOG command with MODE QUIESCE. Let us further assume that UOR A and UOR B are both actively changing the data and will not commit till some future times T1 and T2 respectively.

In this scenario, if we issue an ARCHIVE LOG MODE(QUIESCE) command at time T0, then the following actions are taken.

All NEW UPDATE REQUESTS (can be insert, delete or update) activity to the system is suspended
System waits for current Active UOWs to commit or rollback. Once a UOW has committed or rolled back, it too is not allowed to do any further updates. This process is repeated till all current Active UOWs have been brought to a consistent state and there is NO UPDATE ACTIVITY going on in the system or the QUIESCE TIMEOUT (ZPARM) parameter is reached.
If the system is successfully quiesced a system wide check point is taken and recorded in the log and the BSDS.
The log offload task is initiated and the control returned to the user or the job that issued the command

It should be evident by now that the log we just archived will contain all the relevant records that pertain to UOW A and UOW B. If this log is used at the DR site, then we can successfully recover to a consistent point in time because the archived log DOES NOT CONTAIN ANY INCOMPLETE UOWs.