Example: illustration of consistency group time

The example in Figure 1 further illustrates the concept of consistency group time. The frozen point-in-time in the example shows how the system data mover (SDM) processes successive groups of volume updates. The SDM reads data updates from the primary volumes, groups them into time intervals, and then writes them to the journal and secondary volumes. The word Done in a column means that the action has completed.

Figure 1. A point-in-time look at the XRC volume synchronization process

 
           Group      Read from       Write to         Write to
                       Primary        Journal          Secondary
          ----------------------------------------------------------
 
             1          Done            Done             Done
 
             2          Done            Done         Not yet started
 
             3          Done         In process
 
             4       In process

The following are definitions for the column headings in Figure 1:

Group: This collective information set represents all of the write updates that XRC has received from all of the primary volume’s storage controls during a given time interval.
Read from Primary: A "Done" status indicates that the SDM has read the group into its storage.
Write to Journal: A "Done" status indicates that the SDM has successfully written the group onto the journal data sets, which are accessible from the recovery site along with the control data set.
Write to Secondary: A "Done" status indicates that the SDM has completed all update write operations on the secondary disk device for that group’s time interval.

Examples: These examples show the consistency group time that is reported for each of the following commands issued at the point in time that is shown in the example in Figure 1.

If you issue an XQUERY, XSUSPEND, or XEND command, XRC would report the session’s consistency group time as the Group 1 timestamp. Group 1 is the last group of updates that XRC successfully wrote to the secondary volumes.
If you issue an XDELPAIR command to a volume pair that is in the session, XRC reports the consistency group time for the last group of updates that XRC wrote to the secondary volume. If the volume pair is still active in the session, the returned consistency group time is the same as the XRC session consistency group time.
If you were to issue an XRECOVER command at this point in time, the secondary disk device would be consistent up to the Group 2 timestamp. That is the last completed group of interval data that is secured on the journal. Write updates from Groups 3 and 4 would be lost, and considered "bytes in transit". In this case, the data exposure would be the time delta between Group 4 (data written to the primary) and Group 2 (data secured on the journal). Updates will only be applied to secondary volumes if the consistency group timestamp is less than or equal to the earliest interlocked session's journal time.

Note: For a coupled session that was not ended or suspended by an XRC command, the starting consistency_group_time reported is the timestamp for the last known update for the session when the following conditions exist:

A session status is noninterlocked
Updates were not occurring when the session ended (the session was idle).

If the above conditions exist, the consistency time reported may be earlier than the master recovery time or the last session consistency time indicated in the output of an XQUERY command.