494 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
A dependency relationship exists between a particular target and all newer targets (up to and
including a target that shows 100% progress) that share the source until all data is copied to
this target and all older targets.
For more information about Multiple Target FlashCopy, see 10.3.6, “Interaction and
dependency between multiple target FlashCopy mappings” on page 498.
10.3.3 Consistency Groups
Consistency Groups address the requirement to preserve point-in-time data consistency
across multiple volumes for applications that include related data that spans multiple
volumes. For these volumes, Consistency Groups maintain the integrity of the FlashCopy by
ensuring that “dependent writes” are run in the application’s intended sequence.
When Consistency Groups are used, the FlashCopy commands are issued to the FlashCopy
Consistency Group, which performs the operation on all FlashCopy mappings that are
contained within the Consistency Group at the same time.
Figure 10-5 shows a Consistency Group that includes two FlashCopy mappings.
Figure 10-5 FlashCopy Consistency Group
Dependent writes
To show why it is crucial to use Consistency Groups when a data set spans multiple volumes,
consider the following typical sequence of writes for a database update transaction:
1. A write is run to update the database log, which indicates that a database update is about
to be performed.
2. A second write is run to perform the actual update to the database.
Important: After an individual FlashCopy mapping is added to a Consistency Group, it can
be managed as part of the group only. Operations, such as prepare, start, and stop, are
no longer allowed on the individual mapping.
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