Comment by cryptonector

Say you have mirrored devices. Or RAID-5, whatever. Say the devices don't lie about flushing caches. And you fsync(), and then power fails, and on the way back up you find data loss or worse, data corruption. The devices didn't fail. The OS did.

One need not even assume no device failure, since that's the point of RAID: to make up for some not-insignificant device failure rate. We need only assume that not too many devices fail at the same time. A pretty reasonable assumption. One relied upon all over the world, across many data centers.