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

Jeff Moyer
2015-11-02T21:02:56Z

Dave Chinner <david@fromorbit.com> writes:

> On Mon, Nov 02, 2015 at 09:22:15AM -0500, Jeff Moyer wrote:
>> Dave Chinner <david@fromorbit.com> 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
> -> ELEVATOR_INSERT_FLUSH -> REQ_SOFTBARRIER and subsequent IO
> scheduler calls to elv_dispatch_sort() that don't pass
> REQ_SOFTBARRIER in the queue.

This part is right.

> 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.

But this part is not. It is up to the I/O scheduler to decide when to
dispatch requests. It can hold on to them for a variety of reasons.
Flush requests, however, do not go through the I/O scheduler. At the
very moment that the flush request is inserted, it goes directly to the
dispatch queue (assuming no other flush is in progress). The prior
requests may still be waiting in the I/O scheduler's internal lists.

So, any newly dispatched I/Os will certainly not get past the REQ_FLUSH.
However, the REQ_FLUSH is very likely to jump ahead of prior I/Os in the
queue.

> 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...

Des xfs rely on this model for correctness? If so, I'd say we've got a
problem.

Cheers,
Jeff