Re: Plan: journalling fixes for WAPBL

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

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.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--