Setting Control Module Cache Modes

Each control module contains a battery-backed cache. The active control module operates in one of two modes:

• Write-back. Data is stored in the cache until it is written to disk, which may improve performance. The cache battery protects the data. In a dual control module configuration, when write-back mode is enabled, cache data is also mirrored across the two controllers, providing additional data protection.
• Write-through. Data is written immediately to disk. This reduces the risk of data loss, but may have a negative impact on performance.

You can set policies that control whether a cache will operate in write-back or write-through mode when a cache battery voltage is low or when a control module fails in a dual control module configuration.

Note: It is recommended that you use the default cache policies, described below.

To display the control module cache policies, click:

Members > member_name > Controllers tab

The Member Controllers window appears (Figure 14: Member Controllers ).

In the Memory Cache panel, choose the desired cache mode policies:

• Single-controller-safe policy. To enable, select Use write-through mode if only one controller is functional.

If enabled on a member with a single control module, the control module cache will always operate in write-through mode. If disabled (the default), under normal conditions, the cache will operate in write-back mode.

If enabled on a member with dual control modules, but only one module is functioning, the cache on the functional control module will operate in write-through mode. If disabled (the default), the cache on the functional control module will continue to operate in write-back mode.

• Low-battery-safe policy. To enable, select Use write-through mode if battery charge is below tolerance.

If enabled (the default) on a member with a single control module, and the charge on the cache battery is low, the cache will operate in write-through mode.

On a member with dual control modules, if the low-battery-safe policy is enabled (the default) and the single-controller-safe policy is disabled, the active control module cache will continue to operate in write-back mode if the cache battery has an adequate charge, even if the secondary control module’s cache battery has a low battery charge. If both the low-battery-safe and single-controller-safe policies are enabled, the active control module cache will operate in write-through mode if the cache battery charge on either control module is low.

If the low-battery-safe policy is disabled, a cache will operate in write-back mode, regardless of cache battery charge. Disabling low-battery-safe mode is not recommended because of the risk of data loss.

After choosing the cache policies, click Save changes. If you make a mistake, click Discard changes.