set to Disable. All I/O to and from the local storage system (either Enable or Disable) always uses cache mode. Write operations are always backed up in duplex cache.

◦If you select Enable, after receiving the data into the local storage system cache memory , the local storage system signals the host that an I/O operation has completed and then asynchronously destages the data to the external storage system.

◦When emulating OPEN volumes and selecting Disable, the local storage system signals the host that an I/O operation has completed only after the local storage system has synchronously written the data to the external storage system. When emulating mainframe volumes, when you select Disable, after receiving the data into the local storage system cache memory, the local storage system signals the host that an I/O operation has completed and then asynchronously destages the data to the external storage system.

If you perform the Cache Residency Manager operation on an external volume with Cache Mode set to Disable, the bind mode of Cache Residency Manager cannot be specified. For Cache Residency Manager operations, see the HP StorageWorks XP24000/XP20000 Cache Residency Manager User Guide.

When you set the cache mode, note the following:

◦Data that is not written by the host (for example, data written by HP StorageWorks XP Continuous Access Software) is asynchronously destaged to the external storage system regardless of the Cache Mode setting.

◦If you set the emulation type for a mainframe system (such as 3390-x), data that is written by a host using a command such as Format Write is asynchronously destaged to the external storage system regardless of the Cache Mode setting. Data that is written by a host using a command such as Update Write is destaged to the external storage system as configured in the Cache Mode setting.

•Inflow Control (Enable or Disable)

Inflow control specifies whether the writing operation to the cache memory is stopped (Enable) or continued (Disable) when the writing operation to the external volume is impossible. By default, inflow control is set to Disable.

◦If you select Enable, the writing operation to cache is stopped and the I/O from the host is not accepted when the writing operation to the external volume is impossible.

◦If you select Disable, the I/O from the host during the retry operation is written to the cache memory even after the writing operation to the external volume is impossible. Once the writing operation to the external volume becomes normal, all the data in the cache memory is written to the external volume (all the data is destaged).

•CLPR

When the cache memory is partitioned using HP StorageWorks XP Disk/Cache Partition Software, configure a cache logical partition (CLPR) to access the mapped volume. It is strongly recommended that you place External Storage array groups in a CLPR other than CLPR0. See the HP StorageWorks XP24000/XP20000 Disk/Cache Partition User Guide for detailed information on CLPR.

Cross-subsystem Paths

A cross-subsystem path is a route from a local storage system port to an external storage system port. To prepare for possible failures of the cable, the switch, or the channel processor, HP recommends that you create redundant cross-subsystem paths. This redundancy allows you to continue performing the I/O operations to the external volumes when you maintain the cable. You can set up to eight paths.

18 About XP External Storage Operations