Re: [dm-devel] [PATCH 2/5] dm: implement REQ_FLUSH/FUA support for bio-based dm

[Mike Snitzer <snitzer redhat com>]

On Wed, Sep 01 2010 at  9:50am -0400,
Tejun Heo <tj kernel org> wrote:

> Hello,
> 
> On 09/01/2010 03:43 PM, Mike Snitzer wrote:
> > On Mon, Aug 30 2010 at  5:58am -0400,
> > Tejun Heo <tj kernel org> wrote:
> > 
> >> This patch converts bio-based dm to support REQ_FLUSH/FUA instead of
> >> now deprecated REQ_HARDBARRIER.
> >>
> >> * -EOPNOTSUPP handling logic dropped.
> > 
> > Can you expand on _why_ -EOPNOTSUPP handling is no longer needed?  And
> > please at it to the final patch header.
> 
> It just doesn't happen anymore.  If the underlying device doesn't
> support FLUSH/FUA, the block layer simply make those parts noop.  IOW,
> it no longer distinguishes between writeback cache which doesn't
> support cache flush at all and writethrough cache.  Devices which have
> WB cache w/o flush very difficult to come by these days and there's
> nothing much we can do anyway, so it doesn't make sense to require
> everyone to implement -EOPNOTSUPP.
> 
> One scheduled feature is to implement falling back to REQ_FLUSH when
> the device advertises REQ_FUA but fails to process it, but one way or
> the other, the goal is encapsulating REQ_FLUSH/FUA support in block
> layer proper.  If FLUSH/FUA can be retried using a different strategy,
> it should be done inside request_queue proper instead of pushing retry
> logic to all its users.

OK, so maybe add this info to the patch header one of the primary
FLUSH+FUA conversion patches?

Thanks for the detailed explanation!

Mike