That's probably in cases where the disk controller can't guarantee that accepted writes will make it to the disk in the case of power failure. I worked with bar

That's probably in cases where the disk controller can't guarantee that accepted writes will make it to the disk in the case of power failure. I worked with bare metal servers that had redundant power supplies including to the caching/raid disk controller and disks that in the event of power loss could still guarantee that writes that were 'sync'd by software were written after power loss.