I am wondering what's the purpose of a BBU. My first understanding was, that it enables the cache to write the data to the disc during a power failure. But some specifications say that a BBU can hold its data for up to 72h. I'd expect the data to be written to the disc within milliseconds (given, that the disc still has power, too).
So should a BBU not just protect the cache, but the whole disc for some seconds, too? Wouldn't that be even more secure, because the cache data is written to the disc instead of being around in the cache and waiting for power again? After a second or so, the disc could be shut down.
Best Answer
It doesn't power the disks, it just keeps the data in the cache for (in this case) up to 72 hours until you bring the machine back on line. When you power the machine back up it will write the contents of the cache back out to the disks.
All it does is protect against a power failure. If (for some reason) the machine loses power without cleanly flushing the data out to disk the battery keeps the cache contents alive until you can restart the machine.
It is not a UPS for disks, as the disks could be in an external disk array, or even on a different power circuit. Even a UPS could fail.