Re: [PATCH v7 0/6] block: Introduce REQ_ALLOCATE flag for REQ_OP_WRITE_ZEROES

[Christoph Hellwig]

On Wed, Mar 25, 2020 at 09:26:56AM -0700, Darrick J. Wong wrote:
> > That said, I do think that we have traditionally put emphasis on the
> > wrong part of these operations. All we ever talk about wrt. discard and
> > friends is the zeroing aspect. But I actually think that, semantically,
> > the act of allocating and deallocating blocks is more important. And
> > that zeroing is an optional second order effect of those operations. So
> > if we could go back in time and nuke multi-range DSM TRIM/UNMAP, I would
> > like to have REQ_OP_ALLOCATE/REQ_OP_DEALLOCATE with an optional REQ_ZERO
> > flag. I think that would be cleaner. I have a much easier time wrapping
> > my head around "allocate this block and zero it if you can" than "zero
> > this block and do not deallocate it". But maybe that's just me.
> 
> I'd love to transition to that.  My brain is not good at following all
> the inverse logic that NOUNMAP spread everywhere.  I have a difficult
> time following what the blockdev fallocate code does, which is sad since
> hch and I are the primary stuckees^Wmeddlers^Wauthors of that function. :/

I am very much against that for the following reason:

 - the current REQ_OP_DISCARD is purely a hint, and implementations can
   (and do) choose to ignore it
 - REQ_OP_WRITE_ZEROES is an actual data integrity operation with
   everything that entails

Going back to mixing these two will lead to a disaster sooner or later.