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Chapter 17. Global Copy overview 203
17.2 Volume states and change logic
Figure 17-2 illustrates the basic states and the change logic of a volume that is in either a
Metro Mirror or Global Copy relationship. The following considerations apply to the volume
states when the pair is a Global Copy pair:
Simplex: The volume is not in a Global Copy relationship.
Suspended: In this state, the writes to the primary volume are not mirrored onto the
secondary volume. The secondary volume becomes out-of-sync. During this time, Global
Copy keeps a bitmap record of the changed tracks in the primary volume. Later, the
volume pair can be restarted, and then only the tracks that were updated are copied.
Full duplex: A Global Copy volume never reaches this volume state. In the full duplex
condition, updates on the primary volumes are synchronously mirrored to the secondary
volume—with Metro Mirror, this is possible. With Global Copy, however, even when no
tracks are out of sync, the pair remains in copy pending status.
Copy Pending: Updates on the primary volume are non-synchronously mirrored to the
secondary volume.
Figure 17-2 Global Copy and Mirror state change logic
In regard to the state change logic, the following considerations apply:
When you initially establish a mirror relationship from a volume in simplex state, you have
the option to request that it becomes a Global Copy pair (see the Establish Global Copy
arrow in Figure 17-2), or a Metro Mirror pair (see the Establish Metro Mirror arrow in
Figure 17-2).
Pairs can change from the copy pending state to the full duplex state when a go-to-SYNC
operation is executed (see the Go to sync arrow in Figure 17-2).
Simplex
Suspend
Copy Pending
Full Duplex
Establish Global
Copy
Terminate
Establish Metro
Mirror
Terminate
Suspend
Suspend
Go to sync and
suspend
Resync
Resync
Go to sync
Terminate