Dell PowerVault NX3500 Administrator's Manual page 72

Setting the Migration Policy
In case of a controller failure, the system automatically migrates each
connection from the failed controller to another controller to allow clients to
continue their work. This causes disconnections to CIFS clients. When the
failed controller revives, the system can rebalance the load, by migrating
clients back to the revived controller automatically. This operation is called
fail-back.
Clients that use NFS are stateless and are not affected during fail-back.
Clients that use connection-based protocols (CIFS), may be disconnected
during fail-back. To optimize the fail-back operation, the system provides you
with the following policies for migration on recovery, which affect the load-
balance and disconnections:
Migrate Immediately
Always keep the system well balanced, at the cost of possibly disconnecting
CIFS clients during work time.
Migrate Automatically
Always keep the system well balanced if the controller failure is very short, at
the cost of disconnecting CIFS clients. This option causes the system to
remain unbalanced for a period of several days, if the failure remains for long
time.
This mode overcomes short controller failures because clients have not
created new material during the short time failure. The best practice is to
therefore rebalance them as soon as possible.
If the failure is longer than 10 minutes, the system remains unbalanced until
you rebalance it manually.
Migrate Manually
Never migrates clients automatically. This requires manual intervention to
rebalance the system.
If the system requires manual intervention to rebalance it after fail-over, the
system sends an appropriate message by email to the administrator.
You can configure the fail-back policy, as described above, per protocol and
LAN/client network.
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Monitoring PowerVault NX3500