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

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Fri, Sep 23, 2016 at 01:02:26PM +0000, Paul.Koning%dell.com@localhost wrote:
> 
> > On Sep 23, 2016, at 5:49 AM, Edgar Fu?? <ef%math.uni-bonn.de@localhost> wrote:
> > 
> >> 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?
> 
> I'm not sure.  But I have the impression that in the real world tagging is rarely, if ever, used.

I'm not sure what you mean.  Do you mean that tagging is rarely, if ever,
used _to establish write barriers_, or do you mean that tagging is rarely,
if ever used, period?

If the latter, you're way, way wrong.

Thor