If you enable Cache Mode, you must define a cache logical partition (CLPR). A CLPR helps to protect performance of the local storage system when the I/O rate exceeds the capabilities of the external storage system.
The Cache Mode setting, which you specify during a mapping operation, and a proper cache configuration affect the performance of the external storage used with the local storage system.
When data is written to a mapped external volume, Cache Mode controls when the write-complete response is sent to the host:
- If Cache Mode is enabled, the write-complete response is sent when the write data is in the cache of the local storage system. This can adversely impact overall performance of the local storage system if the I/O rate exceeds the performance capabilities of the external system. If you enable Cache Mode, you must use the same formula for sizing cache in both the local and external storage systems.
- If Cache Mode is disabled, the write-complete response is sent when the write data is accepted by the external storage system. This is the safest action to take and is recommended when there is a possibility that the I/O rate will exceed the short term capabilities of the external storage.
Cache Mode effects with other Hitachi software
Note the following additional effects regarding the Cache Mode setting:
- Data that is not written by the host (for example, data written by ShadowImage) is asynchronously destaged to the external storage system regardless of the Cache Mode setting.
- When you enable Cache Mode, consider
the system load. Normally when you enable Cache Mode, write response or host
write performance improves because data written by the host is applied to the
external storage system asynchronously by using the local storage system's
cache.
However, if the system load is high (for example, cache usage rate exceeds 60%), the local storage system suppresses write operations from the host in order to lower the load. Because of this, even if Cache Mode is enabled, write responses from the host might become slower or host write performance might become lower than when the cache mode is disabled.
- (VSP 5000 series) When you set emulation type for a mainframe system, note the following:
- Data written by a host using a Format Write command is asynchronously destaged to the external storage system regardless of the Cache Mode setting.
- Data written by a host using other Write commands are destaged to the external storage system as configured in the Cache Mode setting.
- (VSP E series) The external volumes in a Dynamic Provisioning pool must all use the same Cache Mode setting, either Enabled or Disabled.
- (VSP E series) Dynamic Tiering pool volumes require Cache Mode to be enabled.