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Distributed devices in a consistency group use the rule set applied to the consistency group. If the distributed device will not
be in a consistency group, you must select a rule set for the distributed device. You can change the rule set in the distributed
device properties dialog box at any time. Also, you can use the CLI to create your own detach rule for the distributed device (as
long as the device is not in a consistency group). For more information about creating and managing detach rules, see the CLI
Guide for metro node.
Data synchronization
You can indicate whether to perform an initial data synchronization from the source device to the target device or not. It is
an initial data synchronization. If you choose not to perform an initial synchronization, future writes will still be mirrored. If the
source device contains data you want to preserve, then synchronize the data. If the device does not contain data or needs
formatting, then you do not need to synchronize.
If the device has a virtual volume, it is automatically synchronized. Any data on the target device will be overwritten.
Logging volumes
Logging volumes are created during initial system setup and are required in each cluster to keep track of any blocks written
during a loss of connectivity between clusters. After an inter-cluster link is restored, the logging volume is used to synchronize
distributed devices by sending only changed blocks over the inter-cluster link. The CLI Guide for metro node provides more
information about logging volumes.
To create a distributed device, a logging volume must exist in the system. If no logging volume exists in the system, an error
message appears to inform you that you cannot create a distributed device without a logging volume. In the GUI, the logging
volume is automatically selected for the distributed device. To specify a logging volume to be used for a distributed device, use
the CLI.
Distributed device rule sets
Rule sets are predefined rules that determine which cluster continues I/O during an inter-cluster link failure or cluster failure. In
these situations, until communication is restored, most I/O workloads require specific sets of virtual volumes to resume on one
cluster and remain suspended on the other cluster.
In a metro node Metro configuration, there is a Management Console on the management server in each cluster. You can create
distributed devices by using the GUI or CLI on either management server. During communication problems, the default rule set
makes the cluster that is used to create the distributed device the winner cluster. I/O proceeds on the winner cluster and is
suspended on the loser cluster. The following table describes how I/O proceeds when you use the default rule sets:
Rule set
Cluster-1 Cluster-2
Cluster-1 detaches Services I/O Suspends I/O
Cluster-2 detaches Suspends I/O Services I/O
NOTE: Rule sets are applied per distributed device.
You can change a distributed device's default rule set in the Distributed Device Properties dialog box if the distributed device
is not in a consistency group. The CLI Guide for metro node provides detailed information about creating and managing rule sets.
Changing the rule set for a distributed device
About this task
By default, the GUI applies the cluster-1-detaches rule set to all distributed devices, however, you can change this rule set after
creating the device as described in the following procedure. When a cluster detaches, I/O continues on the detached (winning)
cluster. Distributed device rule sets provides more information about rule sets.
NOTE: You cannot use this option to change the rule set of a distributed device that is in a consistency group.
82 Provisioning storage