IBM Storwize V5010 Manual page 653

A site attribute must be set for any host, Storwize V5000 storage system, and external
virtualized storage system. The host uses the local I/O group (same site attribute) for data
access, as shown in Figure 10-157.
The continuous blue line shows the host default access path to the volume at the same site.
The dotted blue line shows the non-preferred access path that is used if the preferred access
path is not available. Accessing both I/O groups doubles the number of paths from host to
volume. Take note of the limited number of supported paths for your multipath device driver
and limit the number of paths to an acceptable level.
Figure 10-157 Data access
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Chapter 10. Copy services