Raid Controller Batteries

What are these ?

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. 

• If you have a hardware RAID controller, such as the Intel RS3DC080, which features a hardware cache buffer - typically 512MB to 2GB in size, then this applies.

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.

What they do:

It allows the raid card to remember what is in its buffers ( that hasnt been sync'd to disk )

Its very important for people who need high data integrity.. Or to save your DB from certain types of corruption..

(Basically whats on disk, is on disk - so thats safe.. The problem is when the OS thinks its on disk but its actually not and in a RAID card buffer).

In read and write cache mode (also kown as write-back), 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.

Figure 1: Below is an example of one of these Raid batteries; HP 381573-001 Battery Pack For HP P400 Raid Controller.