Most Intel add-in RAID controllers have an option to add a battery backup unit for the onboard cache memory.
When the battery is installed, the RAID on Chip processor has the ability to cache data in memory and work on it on the fly while in cache RAM instead of passing all data plus parity bits through to the disks.
A list of the battery backup units and the RAID controller(s) they are compatible with can be found here:
Once the battery backup is installed on the RAID controller or connected to the RAID controller if the battery is remote to it, the actual battery charging process will not run until the server has booted up.
Simply plugging the server in after installing the battery backup will not initiate a charge.
So, for existing servers where the battery backup has been added in, the ability to enable Write Back Mode with BBU Present in the RAID controller’s BIOS will be available, but Write Back Mode will not happen until the battery is charged.
The server will need to be booted up and either idled or tested for at least 24 hours before the battery will be fully charged. Once charged, a reboot will allow the RAID controller to initiate Write Back Mode. Charge status is indicated in the RAID controller’s BIOS information screen during the boot process. It may also be indicated in the RAID Web Console.
For new servers, the point is fairly moot since the battery can charge while the server is being burned in. The final RAID configuration would be set after the burn-in period along with the Write Back Mode setting.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book