Re: Proposal: B_ARRIER (addresses wapbl performance?)

[Jason Thorpe <thorpej%shagadelic.org@localhost>]

On Oct 29, 2008, at 6:57 PM, David Young wrote:

> On Wed, Oct 29, 2008 at 05:51:09PM -0400, Thor Lancelot Simon wrote:
> > I'd like to propose a new buffer flag, B_ARRIER.  This flag would be
> > defined to cause a write barrier at whatever I/O request carries it.
>
> Thor,
>
> I have two questions about this.
>
> How can a disk driver re-order B_BARRIER blocks with respect to other
> blocks on its queue?  I.e., can a driver ever postpone writing a  
> B_BARRIER
> block until after it writes some !B_BARRIER blocks?

We've actually been down this path before.  I had a private tree years  
ago that included B_BARRIER support in both the disk queue code and  
passed it down the scsipi layer to be turned into ordered tags.  The  
only logical implementation is one that honors B_BARRER at all layers  
where queueing happens.

-- thorpej