Re: Plan: journalling fixes for WAPBL

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

On Fri, Sep 23, 2016 at 11:47:24AM +0200, Manuel Bouyer wrote:
> On Thu, Sep 22, 2016 at 09:33:18PM -0400, Thor Lancelot Simon wrote:
> > > AFAIK ordered tags only guarantees that the write will happen in order,
> > > but not that the writes are actually done to stable storage.
> > 
> > The target's not allowed to report the command complete unless the data
> > are on stable storage, except if you have write cache enable set in the
> > relevant mode page.
> > 
> > If you run SCSI drives like that, you're playing with fire.  Expect to get
> > burned.  The whole point of tagged queueing is to let you *not* set that
> > bit in the mode pages and still get good performance.
> 
> Now I remember that I did indeed disable disk write cache when I had
> scsi disks in production. It's been a while though.
> 
> But anyway, from what I remember you still need the disk cache flush
> operation for SATA, even with NCQ. It's not equivalent to the SCSI tags.

I think that's true only if you're running with write cache enabled; but
the difference is that most ATA disks ship with it turned on by default.

With an aggressive implementation of tag management on the host side,
there should be no performance benefit from unconditionally enabling
the write cache -- all the available cache should be used to stage
writes for pending tags.  Sometimes it works.

-- 
  Thor Lancelot Simon	                                     tls%panix.com@localhost

    "The dirtiest word in art is the C-word.  I can't even say 'craft'
     without feeling dirty."	-Chuck Close