Setting Unit Policieswww.3ware.com 113Enabling and Disabling the Unit Write CacheWrite cache is used to store data locally in memory on the drive before it iswritten to the disk drive media, allowing the computer to continue with itsnext task. This improves performance. However, there may be instances whenyou want the computer to wait for the drive to write all the data to disk beforegoing on to its next task. In this case, you must disable the write cache.Write cache can be turned on or off for individual units in both 3DM and 3BMwithout changing the configuration or losing data from the drives.If you have a BBU (Battery Backup Unit) installed on the controller, thebattery preserves the contents of the controller cache memory for a limitedperiod of time (up to 72 hours) in the event of a system power loss. When aBBU is installed, if the battery is not “Ready,” write cache is disabled andcannot be enabled.The unit's StorSave profile can also determine whether the write cache can beenabled or disabled. A warning message will be given if the change is notpermitted due to the StorSave setting and the state of the unit.To enable or disable unit write cache through 3DM1 Choose Management > Controller Settings from the menu bar in 3DM.2 In the Unit Policies section of the Controller Settings page, check theWrite Cache box to enable it for the designated unit.The page refreshes, and a message at the top confirms the change youhave made.If your system has no BBU, a message will caution you about enablingwrite cache.Note: If write cache is enabled, in the event of a power failure, the data in the writecache will be lost if you do not have a Battery Backup Unit (BBU).To avoid a suddenpower failure if you do not have a BBU, it is advisable to have an UninterruptiblePower Supply (UPS).Note: If the Write Cache checkbox is disabled (not selectable), check to see if theunit has degraded. 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.
