Re: Proposal: B_ARRIER (addresses wapbl performance?)

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

On Oct 30, 2008, at 5:17 PM, Thor Lancelot Simon wrote:

> On Thu, Oct 30, 2008 at 01:28:21PM -0700, Bill Stouder-Studenmund  
> wrote:
> > On Wed, Oct 29, 2008 at 05:51:09PM -0400, Thor Lancelot Simon wrote:
> >
> > The problem is that this won't help. Ordered tags will relate the
> > sequencing of commands relative to each other. The journal, however,
> > doesn't care about the relative ordering of operations, it wants to  
> > know
> > when the writes to the journal have hit stable storage.
> >
> > The key problem is that, on SCSI disks with the write cache  
> > enabled, a
> > write command can complete by writing to the cache.
>
> But SCSI disks don't lie like this unless explicitly configured to,  
> and
> with proper use of tags, there is no need to configure them that way;
> there is no performance benefit.

But it's so much safer to simply use FUA.

-- thorpej