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260 IBM System Storage DS6000 Series: Copy Services with IBM System z
Figure 22-15 Start Global Mirror
This start command triggers events that involve all the volumes within the session. This
includes a very fast bitmap management at the local storage disk subsystem, issuing inband
FlashCopy commands from the local site to the remote site, and verifying that the
corresponding FlashCopy operations successfully finished. This all happens at the microcode
level of the related storage disk subsystems that are part of the session, fully transparently
and in an autonomic fashion from the users perspective.
All C volumes that belong to the Global Mirror session comprise the Consistency Group. Now
let´s see some more details about the Consistency Group creation at the remote site.
22.4 Consistency Groups
To achieve the goal of creating a set of volumes at a remote site that contains consistent
data, asynchronous data replication alone is not enough—it must be complemented with
either a kind of journal or a tertiary copy of the secondary volume. With Global Mirror this third
copy is naturally created through the use of FlashCopy.
The microcode automatically triggers a sequence of autonomic events to create a set of
consistent data volumes at the remote site. We call this set of consistent data volumes a
Consistency Group. The following sections describe the sequence of events that create a
Consistency Group.
22.4.1 Consistency Group formation
The creation of a Consistency Group requires three steps that are internally processed and
controlled by the microcode. Outside of the Licensed Internal Code (LIC), these steps are
fully transparent and do not require any other external code invocation or user action.
The numbers in Figure 22-16 on page 261 illustrate the sequence of the events involved in
the creation of a Consistency Group. This illustration provides only a high level view that is
sufficient to understand how this process works.
Global Copy
Primary
Primary
A
A
Primary
Primary
Primary
Primary
A
B
O
O
S
Host
Remote site
Local site
Primary
Secondary
FlashCopy
C
MODE(ASYNC)
Tertiary
T
B
M
S
B
M
CR: Change recording bit map
Start Global Mirror session
C
R
PENDING
C
R
PENDING
01