Re: XFS journal write ordering constraints?

[Dave Chinner <david@fromorbit.com>]

On Tue, Jun 13, 2017 at 10:14:10AM -0400, Sweet Tea Dorminy wrote:
> Thank you! I'm glad that we've established it's a mismatch between our
> device's implementation and XFS expectations.
> 
> >.... XFS issues log writes with REQ_PREFLUSH|REQ_FUA. This means
> >sequentially issued log writes have clearly specified ordering
> >constraints. i.e. the preflush completion order requirements means
> >that the block device must commit preflush+write+fua bios to stable
> >storage in the exact order they were issued by the filesystem....
> 
> That is certainly what REQ_BARRIER did back in the day. But when
> REQ_BARRIER was replaced with separate REQ_FUA and REQ_FLUSH
> flags, and barrier.txt got replaced with writeback_cache_control.txt,
> the documentation seemed to imply the ordering requirement on *issued*
> IO had gone away (but maybe I'm missing something).

Yes, that's my understanding, too, but I also thought that multiple
outstanding flushes are ordered by the block layer. i.e. flushes can
be reordered against other operations, but not other flushes.  I
could very well be wrong, but flush-to-flush ordering was what I
thought the ordered pending flush list for PREFLUSH requests in
blk_flush_complete_seq() did.

Like I said, Christoph is the expert here - he'll correct me if I'm
wrong.

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com