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

2016-09-24 Thread Thor Lancelot Simon 
On Fri, Sep 23, 2016 at 01:02:26PM +, paul.kon...@dell.com wrote:
> 
> > On Sep 23, 2016, at 5:49 AM, Edgar Fu??  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

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

2016-09-23 Thread Paul.Koning 

> On Sep 23, 2016, at 5:49 AM, Edgar Fuß  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.

paul

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

2016-09-23 Thread David Holland 
On Fri, Sep 23, 2016 at 11:49:50AM +0200, Edgar Fu? 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?

You can have more than one in flight at a time. Typically the more you
can manage to have pending at once, the better the performance,
especially with SSDs.

-- 
David A. Holland
dholl...@netbsd.org

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

2016-09-23 Thread 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?