After the master storage unit has coordinated a consistent point across the source
volumes of the Global Mirror pairs (the B volumes in a Metro/Global Mirror
configuration), the master storage unit monitors the progress of the transfer of the
consistency group to the remote site. When the consistency group transfer has
completed, the master storage unit then establishes the FlashCopy relationships at
the remote site.
In a Global Mirror environment, some rare error cases can occur where the
FlashCopy operations on the remote site are interrupted. The consistency group is
protected and can be recovered. If the site where Global Mirror source volumes
reside is still operational, then Global Mirror will enter a fatal state to preserve the
consistency group. In a Metro/Global Mirror environment, this site is the
intermediate site.
If the site (also the intermediate site in a Metro/Global Mirror environment) where
the Global Mirror source volumes reside is not operational because of a disaster,
the consistency groups on the remote site might only be partially formed. In either
case, you must verify the consistency group on the remote site and determine
whether the changes need to be “rolled forward” (committed) or “rolled
backward” (discarded).
Consistency group tunable values
As part of the consistency group formation, the master storage unit must
communicate with the subordinate storage units to determine a consistent data
point across volumes. Point-in-time copies are written to the tertiary volumes, the
FlashCopy target volumes at the remote site. Figure 14 provides an illustration of
these values.
User-specified values for the following parameters control how often consistency
groups are formed.
Consistency group interval time
The amount of time that the master storage unit can spend coordinating
with the subordinates to form a consistent data point. It is the amount of
time that Global Mirror is in Global Copy mode between the formation of
Consistency
Group Interval
Coordination
Interval
Prepare
FlashCopy
Consistency
Group Interval
Commit
FlashCopy
Consistent Data Drains
to Remote Storage Unit
Consistency Group Drain Time
f2c00541
Figure 14. Forming consistency groups according to specified intervals
124 DS8000 User’s Guide
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