Re: Questions on block drivers, REQ_FLUSH and REQ_FUA

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, May 24, 2011 at 06:32:20PM -0400, Vivek Goyal wrote:
> On Tue, May 24, 2011 at 10:29:09PM +0100, Alex Bligh wrote:
> 
> [..]
> > Q3: Apparently there are no longer concepts of barriers, just REQ_FLUSH
> > and REQ_FUA. REQ_FLUSH guarantees all "completed" I/O requests are written
> > to disk prior to that BIO starting. However, what about non-completed I/O
> > requests? For instance, is the following legitimate:
> > 
> >        Receive        Send to disk         Reply
> >        =======        ============         =====
> >        WRITE1
> >        WRITE2
> >                                            WRITE2 (cached)
> >        FLUSH+WRITE3
> >                       WRITE2
> >                       WRITE3
> >                                            WRITE3
> >        WRITE4
> >                       WRITE4
> >                                            WRITE4
> >                       WRITE1
> >                                            WRITE1
> > 
> > Here WRITE1 was not 'completed', and thus by the text of
> > Documentation/writeback_cache_control.txt, need not be written to disk
> > before starting WRITE3 (which had REQ_FLUSH attached).
> > 
> > >The REQ_FLUSH flag can be OR ed into the r/w flags of a bio submitted from
> > >the filesystem and will make sure the volatile cache of the storage device
> > >has been flushed before the actual I/O operation is started.  This
> > >explicitly guarantees that previously completed write requests are on
> > >non-volatile storage before the flagged bio starts.

Are you writing a bio based driver? For a request based driver request
queue should break down FLUSH + WRITE3 request in two parts. Issue FLUSH
first and when that completes, issue WRITE3.

Thanks
Vivek