Re: Exposing FUA as alternative to DIOCCACHESYNC for WAPBL

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

On Fri, Mar 31, 2017 at 07:16:25PM +0200, Jarom??r Dole??ek wrote:
> > The problem is that it does not always use SIMPLE and ORDERED tags in a
> > way that would facilitate the use of ORDERED tags to enforce barriers.
> 
> Our scsipi layer actually never issues ORDERED tags right now as far
> as I can see, and there is currently no interface to get it set for an
> I/O.

It's not obvious, but in fact ORDERED gets set for writes
as a default, I believe -- in sd.c, I think?

This confused me for some time when I last looked at it.

> I lived under assumption that SIMPLE tagged commands could be and are
> reordered by the controller/drive at will already, without setting any
> other flags.

They might be -- there are well defined mode page bits
to control this, but I believe targets are free to use
whatever default they like.

> 
> > When SCSI tagged queueing is used properly, it is not necessary to set WCE
> > to get good write performance, and doing so is in fact harmful, since it
> > allows the drive to return ORDERED commands as complete before any of the
> > data for those or prior commands have actually been committed to stable
> > storage.
> 
> This was what I meant when I said "even ordered tags couldn't avoid
> the cache flushes". Using ORDERED tags doesn't provide on-media
> integrity when WCE is set.

Setting WCE on SCSI drives is simply a bad idea.  It is
not necessary for performance and creates data integrity
isues.

> Now, it might be the case that the on-media integrity is not the
> primary goal. Then flush is only a write barrier, not integrity
> measure. In that case yes, ORDERED does keep the semantics (e.g.
> earlier journal writes are written before later journal writes). It
> does make stuff much easier to code, too - simply mark I/O as ORDERED
> and fire, no need to explicitly wait for competition, and can drop e.g
> journal locks faster.
> 
> I do think that it's important to concentrate on case where WCE is on,
> since that is realistically what majority of systems run with.

I don't believe most SCSI drives are run with WCE on.

I agree FUA or its equivalent is needed for non-SCSI
drives.

Thor