What should we do about the nvme atomics mess?

[Ming Lei]

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.

> 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`.



Thanks,
Ming