Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jan 25 2011 at  3:46pm -0500,
Vivek Goyal <vgoyal@redhat.com> wrote:

> On Sun, Jan 23, 2011 at 11:31:33AM +0100, Tejun Heo wrote:
> > On Fri, Jan 21, 2011 at 07:49:55PM -0500, Mike Snitzer wrote:
> > > > + * If the device doesn't have writeback cache, FLUSH and FUA don't make any
> > > > + * difference.  The requests are either completed immediately if there's no
> > > > + * data or executed as normal requests otherwise.
> > > 
> > > For devices without a writeback cache, I'm not seeing where pure flushes
> > > are completed immediately.  But I do see where data is processed
> > > directly in blk_insert_flush().
> > 
> > Yeah, it does.  Pure flushes on a device w/o writeback cache, @policy
> > is zero and blk_flush_complete_seq() will directly proceed to
> > REQ_FSEQ_DONE.
> 
> I see following code in __generic_make_request(). I am wondering if empty
> flushes will be completed here itself if device does not have writeback
> cache.
> 
>                 /*
>                  * Filter flush bio's early so that make_request based
>                  * drivers without flush support don't have to worry
>                  * about them.
>                  */
>                 if ((bio->bi_rw & (REQ_FLUSH | REQ_FUA)) && !q->flush_flags) {
>                         bio->bi_rw &= ~(REQ_FLUSH | REQ_FUA);
>                         if (!nr_sectors) {
>                                 err = 0;
>                                 goto end_io;
>                         }
>                 }

Yes, due to the !q->flush_flags check, empty flushes will complete here
if the device's request_queue doesn't advertise support for FLUSH or FUA
via blk_queue_flush().

Mike