Re: Proposal: B_ARRIER (addresses wapbl performance?)

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

On Fri, Oct 31, 2008 at 10:16:41PM -0700, Bill Stouder-Studenmund wrote:
> On Thu, Oct 30, 2008 at 08:17:04PM -0400, 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.
> > 
> > Never mind that if they have power protection, it's still safe to do so.
> > 
> > So from my point of view, this does precisely what WAPBL needs -- render
> > it just as safe as a non-journalled filesystem, or safer, while radically
> > improving performance.
> 
> Until someone turns off the disk caches. Or more accurately forgets to 
> turn them off. My understanding is that all disks come with caches enabled 
> these days.

I'd like an example of a SCSI (including FC or SAS) disk which shipped with
the write cache enabled.  I have never encountered this.  The entire point
of SCSI-style tagged queueing is to make it unnecessary for performance
reasons.

Thor