IBM System storage DS6000 Series, Redbook

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Chapter 22. Global Mirror overview
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In the following paragraphs we refer to the primary volume as the A volume and to the
secondary volume as the B volume for simplicity.
Global Copy does not immediately copy the data as it arrives to the A volume. Instead, this is
an asynchronous process. As soon as a track is changed by an application write I/O, it is
reflected in the out-of-sync bitmap as with all the other changed tracks. There can be several
concurrent replication processes that work through this bitmap, thus maximizing the utilization
of the high bandwidth Fibre Channel links.
This replication process keeps running until the Global Copy volume pair A-B is explicitly or
implicitly suspended or terminated. Note that a Global Mirror session command, for example,
to pause or to terminate a Global Mirror session, does not affect the Global Copy operation
between both volumes.
At this point data consistency does not yet exist at the remote site.
22.3.4 Introduce FlashCopy
FlashCopy is an integral part of the Global Mirror solution, and now it follows as the next step
in the course of establishing a Global Mirror session; see Figure 22-12.
Figure 22-12 Introducing FlashCopy in the Global MIrror solution
The focus is now on the remote site. Figure 22-12 shows a FlashCopy relationship with a
Global Copy secondary volume as the FlashCopy source volume. Volume B is now both at
the same time, a Global Copy secondary volume and a FlashCopy source volume. In the
same disk subsystem is the corresponding FlashCopy target volume.
Note that this FlashCopy relationship has certain attributes that are typical and required when
creating a Global Mirror session. These attributes are:
򐂰 Inhibit target write: Protect the FlashCopy target volume from being modified by anyone
other than Global Mirror related actions.
Global Copy
Primary
Primary
Primary
A
A
Primary
Primary
O
O
S
Host
Remote site
Local site
Primary
Primary
Primary
Primary
A
B
Secondary
PENDING
PENDING
Write I/O
FlashCopy
C
MODE(ASYNC)
Tertiary
T
B
M
S
B
M
TBM: Target Bit Map
SBM: Source Bit Map