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

Vivek Goyal
2011-01-25T20:46:26Z

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;
}
}

Thanks
Vivek