HP XP External Storage Software - Manual (Page 27)

balancing across paths). For Multi mode, all set paths are used at the same time. The paths are used
to execute I/Os to the external LU, distributing the work load (round-robin processing).
For example, when an external LU volume with an alternate path in Single mode is mapped to an
internal LDEV using External Storage XP, host I/O operations to the external LU via the local array
are normally enabled using the mapped path. If the mapped path is not available (for instance, during
array maintenance or following a failure in the channel processor), the path is switched automatically
to the alternate path (if available). As long as an alternate path is available, host I/O operations
continue as usual, even when an error occurs in the original path.
NOTE:
When the primary path cannot be used for the length of the Path Blockade Watch timer (for example,
180 seconds), the path is switched to an alternate path.
If you have not configured any alternate paths, host I/O operations are suspended when the primary
path becomes unavailable (such as during array maintenance operations or following a failure in the
channel processor).
HP recommends configuring alternate paths for safer operation and increased bandwidth. For
instructions, see
Setting alternate paths for external LUs .
You can set alternate paths when the external LUs are mapped as the internal LDEVs (see Mapping
external LUs (Add LU) ). You can also set alternate paths after completing the mapping operation (see
Setting alternate paths for external LUs ).
Example of an alternate path configuration
Figure 2 shows an example of an alternate path configuration. In Figure 2 , external array ports WWN
A and WWN B are connected to CL1-A and CL2-A, respectively, which are designated as external
ports in the local array. You must specify the port of a different cluster in the local array for the alternate
path, as ports CL1 and CL2 are specified in the figure.
Figure 2 Example of alternate path configuration
Figure 3 shows another example of an alternate path configuration. In Figure 3 , two ports are specified
in the local array and connected to ports in the external array via switches. In this example, two ports
from different XP12000/XP10000/SVS200 clusters are specified in the local array, thereby making
it possible to configure an alternate path for high availability.
External Storage XP user guide for the XP12000, XP10000, SVS200 27