Avago Technologies 3ware SAS 9750-16i4e - Manual (Page 114)

Chapter 7. Configuring Units
102 3ware SATA+SAS RAID Controller Card Software User Guide, Version 10.2
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.
Write cache can be turned on or off for individual units in 3DM2, 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.
To enable or disable unit write cache through 3DM2
1 Choose Management >> Controller Settings from the menu bar in
3DM2.
2 In the Unit Policies section of the Controller Settings page, check the
Write Cache box to enable it for the designated unit.
Figure 58. Write Cache Policy in 3DM2
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).
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.