What should we do about the nvme atomics mess?

[Keith Busch]

On Tue, Jul 08, 2025 at 10:46:06AM +0800, Ming Lei wrote:
> On Mon, Jul 07, 2025 at 08:27:43PM -0600, Keith Busch wrote:
> > On Tue, Jul 08, 2025 at 09:27:06AM +0800, Ming Lei wrote:
> > > On Mon, Jul 07, 2025 at 04:18:34PM +0200, Christoph Hellwig wrote:
> > > > Hi all,
> > > > 
> > > > I'm a bit lost on what to do about the sad state of NVMe atomic writes.
> > > > 
> > > > As a short reminder the main issues are:
> > > > 
> > > >  1) there is no flag on a command to request atomic (aka non-torn)
> > > >     behavior, instead writes adhering to the atomicy requirements will
> > > >     never be torn, and writes not adhering them can be torn any time.
> > > >     This differs from SCSI where atomic writes have to be be explicitly
> > > >     requested and fail when they can't be satisfied
> > > >  2) the original way to indicate the main atomicy limit is the AWUPF
> > > >     field, which is in Identify Controller, but specified in logical
> > > >     blocks which only exist at a namespace layer.  This a) lead to
> > > 
> > > If controller-wide AWUPF is a must property, the length has to be aligned
> > > with block size.
> > 
> > What block size? The controller doesn't have one. Block sizes are
> 
> It should be any NS format's block size.

That requires an artificial reduction to a meaningless value.

> > properties of namespaces, not controllers or subsystems. If you have 10
> > namespaces with 10 different block formats, what does AUWPF mean? If the
> > controller must report something, the only rational thing it could
> > declare is reduced to the greatest common denominator, which is out of
> > sync with the true value reported in the appropriately scoped NAUWPF
> > value.
> 
> Yes, please see the words I quoted from NVMe spec, also `6.4 Atomic Operations`
> mentioned: `NAWUPF >= AWUPF`.

The problem is when Namespace X changes its format that then alters
Namesace Y's reported atomic size. That's unacceptable for any
filesystem utilizing this feature.