What should we do about the nvme atomics mess?

[Ming Lei]

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.

>     various problems because the limit is a mess when namespace have
>     different logical block sizes, and it b) also causes additional
>     issues because NVMe allows it to be different for different
>     controllers in the same subsystem.

The spec mentioned clearly that controller AWUPF should be supported by
any namespace format:

```
Atomic Write Unit Power Fail (AWUPF): This field indicates the size of the write
operation guaranteed to be written atomically to the NVM across all namespaces
with any supported namespace format during a power fail or error condition.
```

So I am wondering why nvme driver can't validate NAWUN against AWUPF?


Thanks, 
Ming