Using Extended Consistency Groups; Overview - HP StorageWorks P9000 User Manual

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 storage systems and
multiple secondary systems. The primary EXCTG is the primary system EXCTG and the secondary
EXCTG is the secondary system EXCTG.
Normally, only the secondary EXCTG requires configuration. However, because the P-VOLs in the
primary EXCTG could be used as S-VOLs if a failure occurs, primary EXCTGs should also be
configured. Configuring both primary and secondary EXCTGs provides the following benefits:
Guaranteed Consistency of Data Update Sequence. Continuous Access Journal Z restores
journal data to S-VOLs 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 example in the next figure, the latest time stamp for each journal is shown as follows:
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Figure 14 EXCTG time stamps
48 Using extended consistency groups
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.