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412 IBM System Storage DS6000 Series: Copy Services with IBM System z
27.1 Data migration
Combining the Copy Service functions Metro Mirror and Global Mirror into an environment
with double cascading makes it possible to maintain data consistency while migrating data.
There are two possibilities for accomplishing data consistency during a migration using an
environment that has double cascading. The first possibility is to shut down all applications at
the local site and let the out-of-sync drain completely. The second possibility is to change the
Global Copy relationships to Metro Mirror, then when the out-of-sync is approaching zero. a
freeze is issued to the changed relationships.
Figure 27-1 Double cascading example
Figure 27-1 shows double cascading with a Metro Mirror relationship from the local to the
secondary, and Global Copy from the secondary to the tertiary site, and from the tertiary to
the remote site. There is a cascading relationship at both the secondary and tertiary site
where the volumes are both secondaries and primaries. In this example, if all applications are
stopped at the local site, then the local, secondary, tertiary, and remote will reach a point after
some time where they are all equal; therefore, they will be consistent and data has
successfully been migrated to the remote site volumes.
In the second approach to provide data consistency during migration, again looking at the
example in Figure 27-1, a freeze is first issued to the Metro Mirror relationship from the local
to secondary site. Since Metro Mirror is running between the local to secondary site, there is
already a synchronous relationship. Therefore, the secondary volumes are consistent with
the local site volumes at this time.
The next step would be to change all the Global Copy relationships to Metro Mirror relations
and then issue a freeze when the out-of-sync has reached zero. When the out-of-sync is fully
drained, then there is a consistent relationship from the secondary to the remote and
migration to the remote is complete. Data migration has sent a snapshot of data to the remote
site so that there is consistency. This process would be used when production cannot be
stopped, since production keeps running at the local site. However, once the freeze is first
issued on the Metro Mirror relationship, from that point forward the remote will only be
A
A
B
B
DS6K DS6K
PPRC-MM
DWDM DWDM
<50 Kms
C
C
D
D
DS6K DS6k
PPRC-GCPPRC-GCPPRC-GC
Local
Secondary
Tertiary
Remote