Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN_FLUSH

[Tao Ma]

On 09/12/2010 07:41 PM, Tejun Heo wrote:
> Hello,
>
> On 09/12/2010 10:38 AM, Tao Ma wrote:
> > According to Documentation/block/barrier.txt,
> > QUEUE_ORDERED_DRAIN
> >          Requests are ordered by draining the request queue and cache
> >          flushing isn't needed.
> >
> >          Sequence: drain =>  barrier
> >
> > QUEUE_ORDERED_DRAIN_FLUSH
> >          Requests are ordered by draining the request queue and both
> >          pre-barrier and post-barrier cache flushings are needed.
> >
> >          Sequence: drain =>  preflush =>  barrier =>  postflush
> >
> > And for loop device, it call fsync in barrier request. See do_bio_filebacked in drivers/block/loop.c
> > bool barrier = !!(bio->bi_rw&  REQ_HARDBARRIER);
> >                  struct file *file = lo->lo_backing_file;
> >
> >                  if (barrier) {
> >                          if (unlikely(!file->f_op->fsync)) {
> >                                  ret = -EOPNOTSUPP;
> >                                  goto out;
> >                          }
> >
> >                          ret = vfs_fsync(file, 0);
> >                          if (unlikely(ret)) {
> >                                  ret = -EIO;
> >                                  goto out;
> >                          }
> >                  }
> >
> > So actually the sync is done in barrier and no extra flush is needed?
> > Or Do I misread the rules in barrier.txt?
>
> Hmmm... maybe the doc was a bit confusing.  Any device which has
> writeback cache should have FLUSH in the queue ordered configuration.
> The loop device used vfs_fsync() for cache flushing and didn't support
> ordered sequence.  As such, it should use draining for request
> ordering and suports FLUSH, so the mode to use is DRAIN_FLUSH.
oh, thanks for the explanation.

Regards,
Tao
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