FUA and TCQ (was: Plan: journalling fixes for WAPBL)

[Edgar Fuß]

> The whole point of tagged queueing is to let you *not* set [the write 
> cache] bit in the mode pages and still get good performance.
I don't get that. My understanding was that TCQ allowed the drive to re-order 
commands within the bounds described by the tags. With the write cache 
disabled, all write commands must hit stable storage before being reported 
completed. So what's the point of tagging with cacheing disabled?