RAID Capacitors and Smart Cache Batteries
These batteries are used to protect the contents of a hardware RAID card's write cache from being lost, in the event of a power-outage.
Some RAID capacitors or cache batteries can be particularly expensive. In additional, many cache batteries have a finite life and you should expect to replace these every 3-5 years or so.
Hardware RAID cards can be configured to use their buffer in one of two ways:
In read and write cache mode, the contents of the write cache will be lost, i.e. not committed to disk, in the event of an unexpected power outage. The optional write cache battery or capacitor can protect the contents of this write cache for a limited period of time, usually 48 hours or less. When power is restored, the RAID controller will commit the contents of the cache to disk.
However, your operating system will still have suffered from an unclean shutdown and there is no guarantee that your file system or application state will be any better than if a RAID cache battery was not fitted.
The above diagram shows that the RAID cache battery only protects a portion of the data flow from application to disk.
How to Turn off Write Caching at the Virtual Drive Level
Disabling Write Cache at the Operating System Level
Many applications or systems attempt to turn off disk caching to provide security in the event of a power loss. For example, a Windows Domain Controller will turn off write caching where it can.
However, when a Hardware RAID card is fitted, this does not guarantee that the hardware write cache either on the controller or the end disks will actually be disabled. Do not rely on this feature to provide data integrity in the event of an unplanned power loss.
We recommend that you turn off Automatic Windows Updates, both on Host Virtualisation servers, and also the Guest servers. Systems should be patched regularly as part of your system maintenance plan, rather than automatically. This ensures that the overall shutdown time when the UPS battery is low, is relatively consistent, ensuring a clean shut-down.