HP XP External Storage Software - Manual (Page 26)

If you select Enable, the local array signals the host that an I/O operation completed after receiving
the data into the local array's cache memory, and then asynchronously destages the data to the
external disk array.
If you select Disable, the local array signals the host that an I/O operation completed only after
the local array has synchronously written the data to the external disk array.
NOTE:
Users should disable cache for low price/performance arrays, such as the HP MSA arrays. The
MSA array ports are slower than the XP FC ports. Disabling cache prevents applications that use
the MSA from consuming significant amounts of XP cache.
NOTE:
As an option, consider using XP Disk/Cache Partition as a recommended best practice for
managing XP cache consumption. See
Managing cache with external storage or contact your
HP account support representative for more information about optimizing cache usage for external
storage.
When IO Suppression mode is set to Enable, Cache Mode changes by default to Disable.
If you use an external LU and set Cache Mode to Disable for Cache LUN XP operations, you cannot
use the Cache LUN XP Bind mode. For more information, see the HP StorageWorks Cache LUN
XP user guide for XP12000/XP10000/SVS200.
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
Preparing for External Storage XP operations 26