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276 IBM System Storage DS6000 Series: Copy Services with IBM System z
When you create a Global Mirror environment that spans multiple storage disk subsystems at
the local site, and probably also at the remote site, then you need to establish communication
paths between the involved local storage disk subsystems.
Figure 23-2 on page 275 shows a symmetrical configuration with a one-to-one mapping. You
have to define the corresponding session, with its number, to all potentially involved LSSs at
the local site. As illustrated in Figure 23-2, there is still no connection between the two local
storage disk subsystems. Therefore, we have to define the corresponding Global Mirror
paths; see Figure 23-3.
Figure 23-3 Decide for Master storage server and start a Global Mirror session
Through the start command, you decide which LSS becomes the Master LSS and
consequently which local storage disk subsystem becomes the Master storage unit. This
Master acts indeed like a server in a client/server environment.
The required communication between the Master storage disk subsystem and the
Subordinate storage disk subsystems is inband, over the defined Global Mirror paths. This
communication is highly optimized, and minimizes any potential application write I/O impact
during the coordination phase to about a few milliseconds. For details see 22.4, “Consistency
Groups” on page 260.
Note that this communication is performed over FCP links. At least one FCP link is required
between the Master storage disk subsystem and the Subordinate storage disk subsystem.
Figure 23-4 on page 277 uses dashed lines to show the Global Mirror paths that are defined
over FCP links, between the Master storage disk subsystem and its associated Subordinate
storage disk subsystems. These FCP ports are dedicated for Global Mirror communication
between Master and Subordinates.
Also shown in Figure 23-4, is a shared port on the Master storage disk subsystem, and
dedicated ports at the Subordinates. Not considering availability, from a communications and
traffic viewpoint only, a link would be sufficient for the traffic between the Master and its
Subordinates. For redundancy, we suggest configuring two links.
Global Co
py
Start Global Mirror session
Primary
Primary
A
A
Primary
PENDING
Primary
Primary
A
A
Primary
PENDING
Primary
Primary
Primary
Primary
A
B
Secondary
C
Tertiary
PENDING
Primary
Primary
Primary
Primary
A
B
Secondary
C
Tertiary
PENDING
Network
Network
01
01
Subordinate
Master
Global Copy
paths
Remote site
Local site