IBM TotalStorage DS300 Best Practices Manual page 460

Important: Assign logical drives within an individual array only to a single host
system in this configuration. Otherwise the access of a second host system to
a logical drive may be interrupted, if the first host system forces a controller
failover for the array.
Fabric attachment with quad paths through interswitch link
In this configuration the two switches build a single fabric through the interswitch
link (ISL) as shown in Figure 8-6 on page 443. They share their zone
configuration.
Attention: Connecting two switches to a single fabric through an interswitch
link raises the impact of configuration failures. Zone configuration data is
automatically updated to all switches which are part of the fabric. With two
separated fabrics, hosts may still have connectivity to the DS400 even if one
fabric fails because of incorrect configuration. Therefore we recommend to
use the configuration described in "Fabric attachment with quad paths without
interswitch link" on page 444 instead of this one, but we discuss this
configuration to give an overview about possible implementations.
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IBM TotalStorage DS300 and DS400 Best Practices Guide