Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Jamie Lokier <jamie@shareable.org>]

Mike Snitzer wrote:
> On Thu, Aug 26 2010 at  6:50pm -0400,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
> > Once we have a need for using request-based DM for something other than
> > multipath we can take a fresh look at implementing rq-based FLUSH+FUA.
> > 
> > Mike
> > 
> > p.s. I know how hard NEC worked on request-based DM's barrier support;
> > so I'm not suggesting this lightly.  For me it just seems like we're
> > carrying complexity in DM that hasn't ever been required.
> 
> To be clear: the piece that I was saying wasn't required is the need to
> for request-based DM to clone a FLUSH to send to multiple targets
> (saying as much was just a confusing distraction.. please ignore that).
> 
> Anyway, my previous email's question still stands.

On a slightly related note: DM suggests a reason for the lower layer, or the
request queues, to implement the trivial optimisation of discarding
FLUSHes if there's been no WRITE since the previous FLUSH.

That was mentioned elsewhere in this big thread as not being worth
even the small effort - because the filesystem is able to make good
decisions anyway.

But once you have something like RAID or striping, it's quite common
for the filesystem to issue a FLUSH when only a subset of the target
devices have received WRITEs through the RAID/striping layer since
they last received a FLUSH.

-- Jamie