12x performance drop on md/linux+sw raid1 due to barriers [xfs]

[Dave Chinner]

On Sun, Dec 14, 2008 at 10:02:05PM +0000, Peter Grandi wrote:
> [ ... ]
> 
> > But - as far as I understood - the filesystem doesn't have to
> > wait for barriers to complete, but could continue issuing IO
> > requests happily. A barrier only means, any request prior to
> > that have to land before and any after it after it.
> 
> > It doesn't mean that the barrier has to land immediately and
> > the filesystem has to wait for this. At least that always was
> > the whole point of barriers for me. If thats not the case I
> > misunderstood the purpose of barriers to the maximum extent
> > possible.
> 
> Unfortunately that seems the case.
> 
> The purpose of barriers is to guarantee that relevant data is
> known to be on persistent storage (kind of hardware 'fsync').
> 
> In effect write barrier means "tell me when relevant data is on
> persistent storage", or less precisely "flush/sync writes now
> and tell me when it is done". Properties as to ordering are just
> a side effect.

No, that is incorrect.

Barriers provide strong ordering semantics.  I/Os issued before the
barrier must be completed before the barrier I/O, and I/Os issued
after the barrier write must not be started before the barrier write
completes. The elevators are not allowed to re-оrder I/Os around
barriers.

This is all documented in Documentation/block/barrier.txt. Please
read it because most of what you are saying appears to be based on
incorrect assumptions about what barriers do.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com