IBM DS6000 - Manual (Page 496)

472 IBM System Storage DS6000 Series: Copy Services with IBM System z
Figure 31-3 Failover/Failback terminology
2. Once the outage is over and you decide to resume replicating data, you have a choice as
to whether to resynchronize from the remote site to the local site or vice versa. This
depends on what is required after updating one of the two sites or even updating both
sites after the suspend action.
a. When you decide to resynchronize from the remote site to the local site, the Failback
command is pointing to the remote volumes. You need a PPRC path from the remote
to the local site before you can perform the corresponding commands. Doing this will
resynchronize both sites with the level of data as it is at the remote site. Only the data
that changed at the remote site, from the moment the replication was suspended and
until the Failover happened, is replicated from remote to local volumes.
Figure 31-3 assumes a Metro Mirror (MM) environment that puts the corresponding
MM pair in PENDING state during resynchronization and once everything is replicated,
the MM pair goes into DUPLEX state.
b. If you want to resynchronize both sites with the data level at the local site, you issue
the Failback command towards the volumes at the local site. This replicates the data
changed since the suspend from the local to the remote site. Also, this resets the
tracks that were potentially changed at the remote site after the Failover.
Figure 31-3 assumes an MM environment that places the MM pair in PENDING state
during the resynchronization phase. Once all changed data is replicated, the MM pair
goes into DUPLEX state.
The option to resynchronize from either site is possible due to the availability of change
bitmaps maintained by the DS6000 or DS8000 at both sites.
Primary
Primary
A
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A
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A
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Primary
A
Failover command
Suspends due to planned
or unplanned event
Primary
Primary
A
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A
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P
Primary
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A
Primary
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A
Secondary
S
Remote DS6000 / DS8000 Local DS6000 / DS8000
Replication direction
Primary
Primary
A
Primary
Primary
A
Primary
Primary
A
Primary
Primary
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Primary
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Primary
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Primary
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P P
P
P
S
S
Resynchronisation
Resynchronisation
PRIMARY SUSPENDED
Failback command
SECONDARY DUPLEX
SECONDARY PENDING
PRIMARY DUPLEX
PRIMARY PENDING
OR Failback command
1.
PRIMARY DUPLEX
PRIMARY PENDING
SECONDARY DUPLEX
SECONDARY PENDING
2.
2.