[PATCH 3/7] block: copy offload support infrastructure

[Martin K. Petersen]

> Native copy offload is not supported for stacked devices.

One of the main reasons that the historic attempts at supporting copy
offload did not get merged was that the ubiquitous deployment scenario,
stacked block devices, was not handled well.

Pitfalls surrounding stacking has been brought up several times in
response to your series. It is critically important that both kernel
plumbing and user-facing interfaces are defined in a way that works for
the most common use cases. This includes copying between block devices
and handling block device stacking. Stacking being one of the most
fundamental operating principles of the Linux block layer!

Proposing a brand new interface that out of the gate is incompatible
with both stacking and the copy offload capability widely implemented in
shipping hardware makes little sense. While NVMe currently only supports
copy operations inside a single namespace, it is surely only a matter of
time before that restriction is lifted.

Changing existing interfaces is painful, especially when these are
exposed to userland. We obviously can't predict every field or feature
that may be needed in the future. But we should at the very least build
the infrastructure around what already exists. And that's where the
proposed design falls short...

-- 
Martin K. Petersen	Oracle Linux Engineering