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Chapter 2 About Universal Replicator Operations 2-15
Hitachi Universal Replicator for IBM /OS User’s Guide
Table 2-2 Specifications of Extended Consistency Groups
Item Specifications
The number of extended consistency groups that can be created Up to four per one storage system
The number of journal groups that can be registered in one
extended consistency group
Up to 16
The following explains configuration of extended consistency groups (i.e.,
primary and secondary EXCTGs). Note the following when configuring
extended consistency groups.
Guaranteed Consistency of Data Update Sequence:
URz restores journal data to secondary data volumes by taking the
following steps. The following procedure guarantees consistency of data
update sequence within an extended consistency group:
a. URz checks the extended consistency group for the time stamps of all
journal data that have not been restored to secondary data volumes,
and then identifies the latest time stamp for each journal group.
In the example shown in
Figure 2-4, the latest time stamp for each
journal group is as follows:
• In Journal group 1, the latest time stamp is 15:00.
• In Journal group 2, the latest time stamp is 15:02.
• In Journal group 3, the latest time stamp is 15:03.
• In Journal group 4, the latest time stamp is 15:04.
b. URz searches for the oldest time stamp from the ones identified in step
a and restores data up to that time to the secondary volumes.
In the example shown in
Figure 2-4, the oldest time stamp is 15:00.
URz restores all data that have a time stamp 15:00 or earlier to the
secondary data volumes.
• For Journal group 1, URz restores all data up to 15:00.
• For Journal group 2, URz restores all data up to 14:02.
• For Journal group 3, URz restores all data up to 14:03.
• For Journal group 4, URz restores all data up to 14:04.