HP StorageWorks External Storage XP user guide for the XP12000, XP10000, SVS200 (T1706-96009, October 2007)

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.

AlternatepathmodesincludeSinglemodeandMultimode.Thealternatepathmode,Singlemodeor

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).

Forexample,whenanexternalLUvolumewithanalternatepathinSinglemodeismappedtoan

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(forexample,

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).

Examp

le of an alternate path conﬁguration

Figur

e2shows an example of an alternate path conﬁguration. In Figure 2,externalarrayportsWWNA

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.

Preparing for External Storage XP operations