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262 IBM System Storage DS6000 Series: Copy Services with IBM System z
Once step 3 is complete, a consistent set of volumes have been created at the secondary
site. This set of volumes, the C volumes, represents the Consistency Group.
At this very moment also, the B volumes and the C volumes are, but only for this brief
moment, equal in their content. Immediately after the FlashCopy process is logically
complete, the primary systems’s microcode is notified to continue with the Global Copy
process. In order to replicate the changes to the A volumes that occurred during the step 1 to
step 3 window, the change recording bitmap is mapped against the empty out-of-sync bitmap,
and from now on, all arriving write I/Os will end up again in the out-of-sync bitmap. From now
on the conventional Global Copy process, as outlined in“Create Global Copy relationship
between local and remote volume” on page 256, continues until the next Consistency Group
creation process is started.
22.4.2 Consistency Group parameters
In the previous section we described the sequence of steps followed once a Consistency
Group create event is triggered. In the first step, the
serialization step, when the Consistency
Group spans over more than one primary disk subsystem, Global Mirror not only serializes all
related Global Mirror primary volumes but also does the coordination with other storage disk
subsystems.
Figure 22-17 Consistency Group tuning parameters
There are three externalized parameters, and their default values can be overridden; see
Figure 22-17:
Maximum coordination time: dictates, for the Global Copy primary volumes that belong to
this Consistency Group, how long a host write I/O may be impacted due to coordination
and serialization activities. This time is measured in milli-seconds (ms). The default is 50
ms.
Maximum drain time: is the maximum time allowed for draining the out-of-sync bitmap
once the process to form a Consistency Group is started and step 1 successfully
completed. The maximum drain time is specified in units of seconds. The default is 30
seconds.
If the maximum drain time is exceeded, Global Mirror will fail to form the Consistency
Group and will evaluate the current throughput of the environment. If this indicates that
another drain failure is likely, Global Mirror will stay in Global Copy mode while regularly
re-evaluating the situation to determine when to start to form the next Consistency Group.
2
Serialize all
Global Copy
primary volumes
Drain data from local to remote site
Perform
FlashCopy
1 3
1 2 31 2 3
Maximum
coordination
time
time
CG interval
drain
time
Maximum