IBM DS8000 - ADDITIONAL INFORMATION, User Manual, Page: 154 / 250

You can also customize tasks for other user-defined actions, such as planned site
switches in which the workload is switched from one site to another. For further
information about GDPS, see GDPS/PPRC V3R2 Installation and Customization Guide
(or GDPS/PPRC HyperSwap Manager V3R2 Installation and Customization Guide) and
GDPS Global Mirror V3R2 Installation and Customization Guide. GDPS is available
from IBM Global Services.
Note: The GDPS Installation and Customization Guides are licensed publications
available only to customers that are licensed for GDPS. If you are not
licensed, you can review the publication at http://www.redbooks.ibm.com/
redpieces/abstracts/sg246374.html?Open and contact IBM for further details
on these offerings.
Incremental resynchronization in a Metro/Global Mirror
configuration
The purpose of the incremental resynchronization function is to help you avoid
having to do a full volume resynchronization between the local and remote sites if
an outage occurs at your intermediate site.
In a three-site Metro/Global Mirror configuration without incremental
resynchronization, if there is an outage at the intermediate site, reestablishing a
disaster recovery environment between the local and remote sites requires a full
volume resynchronization. To maintain data integrity, a full volume copy is
performed because there is no guarantee that data that was written to the local
Metro Mirror volumes was secured at the remote site when the outage occurred.
There is also no guarantee that Global Mirror formed successful consistency groups
of consistent data at the remote site before the outage occurred. During this full
volume resynchronization period, which can take a long time depending on the
amount of data being copied, your data can potentially be susceptible to a loss.
To ensure the retention of your data and restart a disaster recovery environment
running Global Mirror between the local and remote sites, you can establish the
primary Metro Mirror volumes at the local site as the primary volumes to the
Global Copy target volumes at the remote site. This implementation, shown in
Figure 15, gives you the flexibility to establish asynchronous mirroring between the
local and remote site. Consistency groups can continue to form at the remote site,
while retaining data consistency over distance between the two sites.
The incremental resynchronization function involves a change recording
mechanism on the specified primary Metro Mirror volumes in a Metro/Global
FlashCopy
Global Mirror
A
B
C D
Metro Mirror
f2c00666
Local site
Remote site
Figure 15. Incremental Resynchronization implementation
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DS8000 User’s Guide