External volume policy settings and functions

When you map an external volume to an internal volume, mapping policies are used. To change the mapping policies, you need to edit the default mapping settings. You can change default mapping settings without affecting the settings in previously mapped volumes.

• Cache Mode: I/O to and from the local storage system always uses cache. Write operations are always backed up in duplex cache. The Cache Mode setting specifies whether write data from the host is written to the external volume asynchronously (Enable) or synchronously (Disable).

• Enable: After receiving the data into the local system's cache memory, the system signals the host that the I/O operation has completed, and then asynchronously destages the data to the external volume.

• Disable (default): After synchronously writing the data to the external volume, the local system signals the host that an I/O operation has completed.

For additional information, see the topic on cache use in the Universal Volume Manager User Guide.

• Cache Partition: Cache memory can be partitioned by using Virtual Partition Manager to configure a cache logical partition (CLPR) for the mapped volumes. Cache logical partitions are often used to limit cache-use by accessing slower external storage volumes.

We strongly recommend that you place external storage array groups in a CLPR other than CLPR0. For details about CLPR, see Virtual Partition Manager User Guide.

• Inflow Control: When the write operation to the external volume cannot be completed, Inflow Control specifies whether the write operation to cache memory is limited (Enable) or continued (Disable).

• Enable: The write operation to cache is limited and I/O from the host is not accepted. Limiting the write operation prevents the accumulation of data that cannot destage to cache memory.

• Disable (default): I/O sent from the host during the retry operation is written to cache memory. When write operations to the external volume are again possible, data in cache memory is destaged to the external volume.

• Use ALUA as Path Mode: In the local storage system, you can select whether ALUA mode is used as the path mode. If ALUA is supported in the profile information of the external storage, Enable is used by default. Otherwise, Disable is used.

• Load Balance Mode: Select Depends on the selected external volume(s), Normal Round-robin, Extended Round-robin, or Disable as a Load Balance Mode for the external storage system. By default, Normal Round-robin is used. However, when the product name of the storage system is displayed as (generic), Depends on the selected external volume(s) is used by default.

If Single is set as the Path Mode or Disable is set for Use ALUA as Path Mode, Load Balance Mode cannot be specified.

• Depends on the selected external volume(s): If Enable is set for ALUA Settable on the external volume, Normal Round-robin is set for Load Balance Mode automatically. If Disable is set for ALUA Settable, Disable is set for Load Balance Mode automatically.

• Normal Round-robin: I/O is distributed to several paths on which I/O operation is enabled for the external storage system.

• Extended Round-robin: I/O is distributed to several paths on which I/O operation is enabled for the external storage system. For sequential I/O, the external volume is divided into sections at regular intervals. In this case, the same path is used for I/O within the same section which reduces the frequency of I/O distribution.

• Disable: As in Single mode, I/O operation is performed using the path that has the highest priority of all paths on which I/O operation is enabled for the external storage system.

• CLPR: Specify CLPR when accessing a mapped volume if cache memory is split in Virtual Partition Manager. For detailed information, refer to the Virtual Storage Platform G200, G400, G600, G800 Performance Guide.