Is the battery redundant if the server is on a properly-sized UPS? There is an option to force write-back mode and I want to understand the risk. It is my understanding that it helps against power failures while data in the cache has not been written out to physical disk. But if the server is on a UPS, then there is no risk of power loss, especially if it is a smart UPS and the server is configured to shut down gracefully when the battery reaches a low state. Am I missing something?