Re: [PATCH] 2.4.x write barriers (updated for ext3)

Stephen C. Tweedie
2002-03-04T19:58:20Z

Hi,

On Mon, Mar 04, 2002 at 08:48:02PM +0100, Daniel Phillips wrote:
> On March 4, 2002 07:05 pm, Stephen C. Tweedie wrote:
> > Also, as soon as we have journals on external devices, this whole
> > thing changes entirely. We still have to enforce the commit ordering
> > in the journal, but we also still need the ordering between that
> > commit and any subsequent writeback, and that obviousy can no longer
> > be achieved via ordered tags if the two writes are happening on
> > different devices.
>
> But the bio layer can manage it, by sending a write barrier down all relevant
> queues. We can send a zero length write barrier command, yes?

Sort of --- there are various flush commands we can use. However, bio
can't just submit the barriers, it needs to synchronise them, and that
means doing a global wait over all the devices until they have all
acked their barrier op. That's expensive: you may be as well off just
using the current fs-internal synchronous commands at that point.

--Stephen