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3ware® User Guide > Configuring Units > Setting Unit Policies > Enabling and Disabling the Unit Write Cache

Enabling and Disabling the Unit Write Cache
Write cache is used to store data locally in memory on the controller before it is written to the disk drive media, allowing the computer to continue with its next task. This improves performance. However, there may be instances when you want the computer to wait for the drive to write all the data to disk before going on to its next task. In this case, you must disable the write cache.
 
Note: If write cache is enabled, in the event of a power failure, the data in the controller cache will be lost if you do not have a Battery Backup Unit (BBU). To avoid a sudden power failure if you do not have a BBU, it is advisable to have an Uninterruptible Power Supply (UPS).
Write cache can be turned on or off for individual units in 3DM, CLI, and 3BM without changing the configuration or losing data from the drives.
If you have a BBU (Battery Backup Unit) installed on the controller, the battery preserves the contents of the controller cache memory for a limited period of time (up to 72 hours) in the event of a system power loss. When a BBU is installed, if the battery is not “Ready,” write cache is disabled and cannot be enabled.
The unit's StorSave profile can also determine whether the write cache can be enabled or disabled. A warning message will be given if the change is not permitted due to the StorSave setting and the state of the unit.
 
Note: If the Write Cache checkbox is disabled (not selectable), check to see if the unit has degraded or if the BBU is not ready. If a unit has a StorSave policy of “Protect” and the unit degrades, the policy prevents write cache from being re-enabled until the unit has been rebuilt or until the BBU is ready to use.
To enable or disable unit write cache through 3DM
1
Choose Management > Controller Settings from the menu bar in 3DM.
2
In the Unit Policies section of the Controller Settings page, check the Write Cache box to enable it for the designated unit.
The page refreshes, and a message at the top confirms the change you have made.
If your system has no BBU, a message will caution you about enabling write cache.
To enable or disable unit write cache through 3BM
1
An asterisk appears in the left-most column to indicate that it is selected.
2
Tab to the Maintain Unit button and press Enter.
3
On the pop-up menu, select Configure and press Enter.
4
In the Configure Disk Array screen, Tab to the field Write Cache Settings.
Show Unit Write Cache State in 3BM
The current setting—Enabled or Disabled—is shown. (The initial default setting is for write cache to be enabled.)
5
Press Enter to display the choices, use the arrow keys to select the setting you want, and press Enter again to choose it.
6
Tab to the OK button and press Enter to select it.
You return to the main 3BM screen.
7
When you are finished making changes, press F8 to save them and exit 3BM.

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