HP XP7 Storage - Manual (Page 50)

5 Using extended consistency groups
When performing Continuous Access Journal Z operations between multiple primary and secondary
storage systems, the journals must be in extended consistency groups (EXCTGs). This chapter
provides information and instructions for setting up and maintaining journals in EXCTGs.
Overview
An EXCTG is a collection of journals in which data consistency is guaranteed. You register journals
in an EXCTG when performing copy operations between multiple primary systems and multiple
secondary systems. Master journals are registered in primary EXCTGs, and restore journals are
registered in secondary system EXCTGs.
Because the P-VOLs could be used as S-VOLs if a failure occurs, primary EXCTGs should be
configured. Configuring both primary and secondary EXCTGs provides guaranteed consistency
of the data update sequence, and consistency time.
Guaranteed Consistency of Data Update Sequence
Continuous Access Journal Z restores journal data to S-VOLs sequentially, by taking the following
steps:
1.
Checks the EXCTG for the time stamps of all journal data not restored to S-VOLs; then identifies
the latest time stamp for each journal.
In the following example figure, the latest time stamp for each journal is shown as follows:
- In Journal 1, the latest time stamp is 15:00.
- In Journal 2, the latest time stamp is 15:02.
- In Journal 3, the latest time stamp is 15:03.
- In Journal 4, the latest time stamp is 15:04.
Figure 13 EXCTG time stamps
2. Searches for the oldest time stamp from the previous step, then restores data up to that time
to the secondary volumes.
In the example figure, the oldest time stamp is 15:00. Cnt Ac-J Z restores all data that have
a time stamp 15:00 or earlier to the S-VOLs.
- For Journal 1, Cnt Ac-J Z restores all data up to 15:00.
- For Journal 2, Cnt Ac-J Z restores all data up to 14:02.
- For Journal 3, Cnt Ac-J Z restores all data up to 14:03.
- For Journal 4, Cnt Ac-J Z restores all data up to 14:04.
In this way Cnt Ac-J Z prevents newer journals from being restored before older ones to keep
consistency in data update sequence.
50 Using extended consistency groups