Re: [RFC 00/11] DAX fsynx/msync support

From: Dave Chinner
Date: Mon Nov 02 2015 - 15:10:53 EST

On Mon, Nov 02, 2015 at 09:22:15AM -0500, Jeff Moyer wrote:
> Dave Chinner <david@xxxxxxxxxxxxx> writes:
> > Further, REQ_FLUSH/REQ_FUA are more than just "put the data on stable
> > storage" commands. They are also IO barriers that affect scheduling
> > of IOs in progress and in the request queues. A REQ_FLUSH/REQ_FUA
> > IO cannot be dispatched before all prior IO has been dispatched and
> > drained from the request queue, and IO submitted after a queued
> > REQ_FLUSH/REQ_FUA cannot be scheduled ahead of the queued
> > REQ_FLUSH/REQ_FUA operation.
> >
> > IOWs, REQ_FUA/REQ_FLUSH not only guarantee data is on stable
> > storage, they also guarantee the order of IO dispatch and
> > completion when concurrent IO is in progress.
> This hasn't been the case for several years, now. It used to work that
> way, and that was deemed a big performance problem. Since file systems
> already issued and waited for all I/O before sending down a barrier, we
> decided to get rid of the I/O ordering pieces of barriers (and stop
> calling them barriers).
> See commit 28e7d184521 (block: drop barrier ordering by queue draining).

Yes, I realise that, even if I wasn't very clear about how I wrote
it. ;)

Correct me if I'm wrong: AFAIA, dispatch ordering (i.e. the "IO
barrier") is still enforced by the scheduler via REQ_FUA|REQ_FLUSH
scheduler calls to elv_dispatch_sort() that don't pass
REQ_SOFTBARRIER in the queue.

IOWs, if we queue a bunch of REQ_WRITE IOs followed by a
REQ_WRITE|REQ_FLUSH IO, all of the prior REQ_WRITE IOs will be
dispatched before the REQ_WRITE|REQ_FLUSH IO and hence be captured
by the cache flush.

Hence once the filesystem has waited on the REQ_WRITE|REQ_FLUSH IO
to complete, we know that all the earlier REQ_WRITE IOs are on
stable storage, too. Hence there's no need for the elevator to drain
the queue to guarantee completion ordering - the dispatch ordering
and flush/fua write semantics guarantee that when the flush/fua
completes, all the IOs dispatch prior to that flush/fua write are
also on stable storage...


Dave Chinner
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