Re: Exposing FUA as alternative to DIOCCACHESYNC for WAPBL

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

On Wed, Mar 29, 2017 at 11:53:55AM +0200, Edgar Fu? wrote:
> > It needs to do this [flush disc cache after committing journal] because 
> > it needs to make sure that journal data are saved before we save the 
> > journal commit block.
> So the point is to force an order (data before commit block).
> 
> > Implicitly, the pre-commit flush also makes sure that all asynchronously 
> > written metadata updates are written to the media, before the commit makes
> > them impossible to replay.
> So the point is do force an order (metadata before journal).
> 
> > Even SCSI ORDERED tags wouldn't help to avoid the need for cache flushes.
> So why that if the point of the cache flushes is to ensure an order?

It doesn't make sense to me either.  ORDERED tags are required not to complete
until all previously submitted SIMPLE and ORDERED tags have been committed to
stable storage; and if that's not enough, you can, I believe, use a HEAD tag.
Why isn't that good enough?

If the answer is that you're running with WCE on in the mode pages, then
don't do that: use SIMPLE tags for all writes except when you intend a
barrier, and ORDERED when you do.  I must be missing something.

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

	Cry, the beloved country, for the unborn child that is the
	inheritor of our fear.			-Alan Paton