Also, why batteries in a datacenter? When you implement a flush() command at the lowest level you're faced with two choices: 1) actually write to disk, then return from the call, 2) write to some cache/RAM and have just enough battery locally to ensure that you can write it to disk even if all power goes out.
Then there's the other problem of surviving long enough between a power interruption and diesel generators starting up. But this is a smaller problem, rebooting all instances in a datacenter is less bad than losing some data that was correctly flush()ed by software. Bad flush() behaviour can result in errors that cannot be recovered from without a complicated manual intervention (for example if it causes corrupted and unreadable database files).
