b. other FSes which do not have CoW support. ext4 is already being
used for "atomic writes" in the field

We also need raw block device access to work within the constraints
required by the hardware.

quoted

probably want to do it for optimal performance, but requiring it
feeels rather limited.

The application developers we are working with generally prefer an error
when things are not aligned properly. Predictable performance is key.
Removing the performance variability of doing double writes is the
reason for supporting atomics in the first place.

I think there is value in providing a more generic (file-centric) atomic
user API. And I think the I/O stack plumbing we provide would be useful
in supporting such an endeavor. But I am not convinced that atomic
operations in general should be limited to the couple of filesystems
that can do CoW.

-- 
Martin K. Petersen	Oracle Linux Engineering