HP XP Data Shredder Software - Manual (Page 24)

NOTE:
If you set the emulation type of the mainframe system for the mapped LU, host I/O is always
propagated asynchronously to the external LU regardless of the Cache Mode setting.
• CLPR
When using XP Disk/Cache Partition to partition cache memory, set the cache logical partition
(CLPR) used for accessing the mapped LU. You can also specify whether the CU selected at mapping
is 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 .
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.
24 Preparing for External Storage XP operations