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Chapter 23. Global Mirror options and configuration 293
Note that the Failback operation is issued to the B volumes as the primary and the A volumes
as the secondary. This command changes the A volume from its previous primary pending
state to secondary pending and starts the resynchronization of the changes from B to A.
Before doing the Failback operation, ensure that paths are established from the remote site
LSS to its corresponding LSS at the local site. Note that with Fibre Channel links you can
define paths in either direction on the very same FCP link. During the Failback operation the
application stays running at the remote site to minimize the application outage.
Example 23-12 shows a TSO command example for doing the Failback operation.
Example 23-12 Failback operation from B to A before returning to the local site
//* ----------------------- FAILBACK B -> A -------------------- ***
//EPAIRFB EXEC PGM=IKJEFT01
//SYSPRINT DD SYSOUT=*
//SYSTSPRT DD SYSOUT=*
//SYSTSIN DD DDNAME=SYSIN
CESTPAIR DEVN (X'3C00') +
PRIM (X'3C00' 73081 X'00' X'0C') +
SEC (X'2C00' 27131 X'00' X'0C') +
ACTION(FAILBACK) +
ONLINSEC(NO) MSGREQ(NO) CRIT(NO) +
OPTION (XD) CASCADE(NO)
A remark on the ONLINSEC parameter here: with ONLINSEC(NO) a verification is done to
check whether any connected system has the secondary volume, here device number 2C00,
online. If the path group information in the storage disk subsystem reflects that a system has
this volume online, the CESTPAIR command will not be executed. This is a safety precaution
and protects you from overwriting an online volume by accident. On the other side, there may
be a z/VM system, for example, which has this volume also defined, and is IPLed with this
device as online during startup. The TSO commands executed in another z/OS system will
fail the CESTPAIR.
23.7.9 Return to local site
Figure 23-17 Failover from A to B
Global Copy
Primary
Primary
A
Primary
Primary
A
Tertiary
C
Primary
Primary
A
Primary
Primary
A
B
Remote siteLocal site
Host
Primary
Primary
Primary
A
Primary
Primary
A
A
PENDING
Host
Primary
SUSPENDED
Failover A to B