HP StorageWorks XP Remote Web Console Software - Manual (Page 22)

22 Preparing for External Storage XP operations
restricted to the storage management logical partition (SLPR) the CLPR belongs to. For more
information about CLPRs and SLPRs, see the HP StorageWorks XP Disk/Cache Partition user guide.
Alternate paths
When mapping an external LU as an internal LDEV, paths are set from the internal LDEV to the external
LU.
If two or more paths to the external LU are equipped from different clusters, the number of paths you set
when mapping the volume are available. If one path is equipped, only that path is available.
You can set up to eight paths to each external LU, including paths automatically set. 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 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 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 ” on page 86.
You can set alternate paths when the external LUs are mapped as the internal LDEVs (see ” Mapping
external LUs (Add LU) ” on page 73). You can also set alternate paths after completing the mapping
operation (see ” Setting alternate paths for external LUs ” on page 86).
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