22 External Storage XP for the XP12000/XP10000
Alternate paths
When mapping an external LU as an internal XP LDEV, a path (called a mapped path) is set from the
internal XP LDEV to the external LU.
You can set up to eight paths to each external LU (including paths set automatically). Among the paths to
the external LU, the path with the highest priority is called the primary path, and other paths are alternate
paths.
Alternate path modes include Single mode and Multi mode. The alternate path mode, Single mode or
Multi mode, depends on the connected external disk array. For Single mode, only the path with the highest
priority (primary path) is used to execute I/Os to the external LU. When an error occurs in the primary
path, the path with the second highest priority is used (that is, no per-LU dynamic load balancing across
paths). For Multi mode, all of the 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 with an alternate path in Single mode is mapped to an internal XP LDEV
using External Storage XP, host I/O operations to the external LU via the XP12000/XP10000 are normally
enabled using the mapped path. If the mapped path is not available (for instance, during disk 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 disk 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” on page 74.
You can set alternate paths when the external LUs are mapped as the internal XP LDEVs (see ”Mapping
external LUs (Add LU)” on page 60). You can also set alternate paths after completing the mapping
operation (see ”Setting alternate paths for external LUs” on page 74).
Example of an alternate path configuration
Figure 2 shows an example of an alternate path configuration. In Figure 2, the external disk 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 disk array. You must specify the port of a different cluster in the local disk 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 disk array and connected to ports in the external disk array via switches. In this example, two
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