>> Many raid controllers are smart enough to always turn off write caching on the drives, and also disable the feature on their own buffer without a BBU. Add a BBU, and the cache on the controller starts getting used, but *not* the cache on the drives. > > This does not make sense. > Write caching on all hard drives in the last decade are safe because they support a write cache flush command properly. If the card is "smart" it would issue the drive's write cache flush command to fulfill an fsync() or barrier request with no BBU.
You're missing the point. If the power dies suddenly, there's no time to flush any cache anywhere. That's the entire point of the BBU - it keeps the RAM powered up on the raid card. It doesn't keep the disks spinning long enough to flush caches.