IBM System storage DS6000 Series, Redbook

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Chapter 23. Global Mirror options and configuration
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23.3.4 Global Mirror environment topology changes
The topology of a Global Mirror session, for example, the Master SSID, Subordinate SSIDs,
and Subordinate serial numbers, cannot be altered through a pause and resume command
sequence. When you resume the session after a pause and the Global Mirror topology is not
the same as it was at the pause, the start of the session will fail.
If you need to change the topology of your Global Mirror session, you have to stop the
session and start the session again with the new topology structure information in the start
command.
Topology also refers to the list of storage disk subsystems that are subordinates. You define
control paths between the Master and Subordinate LSSs—just one LSS per subordinate disk
subsystem is sufficient. When you define the control paths, you identify the source LSS on
the Master disk subsystem with its corresponding subsystem ID (SSID). The target LSS in the
path definition command points to a corresponding subordinate disk subsystem, and is also
identified by its corresponding SSID. These SSIDs go into the topology specification, which
defines the communication paths between Master and Subordinate storage disk subsystems.
This topology is specified in the SBINFO parameter and is required for the following Global
Mirror operations:
򐂰 Start a Global Mirror session.
򐂰 Stop a Global Mirror session.
򐂰 Pause a Global Mirror session.
򐂰 Resume a Global Mirror session.
In Example 23-1 the topology information is passed to Global Mirror in the context of
establishing the paths between Master and Subordinate.
Example 23-1 Context between establish path and Global Mirror topology
CESTPATH DEVN (X'6000') +
PRIM (X'6000' 500507630EFFFC6F X'00') +
SEC (X'6400' 500507630EFFFCA0 X'00') +
LINK (X'00000000' X'01000100') CGROUP(NO)
RSESSION SNBR(01) VOLSER(AA601F) +
ACTION(START) +
LSSTYPE(CKD) LSSNBR(00) +
MSSERIAL (AAGXA) MASTER(YES) +
SBINFO(6000,6400,AAVCA)
On the CESTPATH command, the sending Master SSID is 6000 and the receiving
Subordinate SSID is 6400 . These are the SSIDs as they are defined in the establish path
command. Also in this command there is a link between the SSID and its corresponding LSS
number, which in both cases is x’00’.
Important: When setting new values for the tuning parameters, be sure to check for errors
in Consistency Group formation and in draining the out-of-sync bitmaps. A few errors are
not significant and do not jeopardize the consistency of your Global Mirror. However, if
failures repeatedly occur, such as, no more consistency groups are formed, or the
percentage of successful Consistency Groups is unacceptable, or the frequency of
Consistency Groups is not meeting your requirements (Recovery Point Objective-RPO),
then the new values set for the tuning parameters need to be revised and changed.