[RFC] block/nvme: exploring asynchronous durability notification semantics

[Esteban Cerutti]

Hi Hannes, Christoph,

thank you both for the detailed replies. They were very helpful and
corrected several misconceptions on my side.

In particular, I now understand that:

- reliance on synchronous flushes is not universal and largely depends
  on filesystem design (e.g. ext4 vs btrfs),
- FUA already provides a per-request durability guarantee when needed,
- and that power-backed guarantees at the system level are much harder
  to make reliable than they may appear.

Based on your feedback, I took some time to read more about how this
problem is handled in practice, especially at the filesystem and
database level (e.g. journaling, WAL, group commit, batching of fsync).

It seems that the cost of durability in fsync-heavy workloads is indeed
well known, but is typically addressed at higher layers rather than in
the block layer or device interface.

So rather than defending my original idea, I would like to refine the
question.

Is there a fundamental reason why these optimizations are handled
exclusively at the filesystem / application level, instead of being
exposed (or assisted) at a lower level such as the block layer or
device interface?

For example, is it due to:

  - lack of reliable visibility into device-internal persistence state,
  - difficulty in defining correct semantics across different hardware,
  - or simply that higher layers already have enough information to
    optimize safely (making lower-level support unnecessary)?

I realize that my earlier proposal likely underestimated the complexity
involved, but I am still trying to understand whether there is a
meaningful limitation at the lower layers, or if this is primarily a
question of where the problem is best solved.

Thanks again for your time and for the insights — this has already been
very useful for understanding how these pieces fit together.

Best regards,
Esteban