Re: [PATCH v7 0/3] FDP and per-io hints

Christoph Hellwig
2024-10-11T09:02:30Z

On Thu, Oct 10, 2024 at 01:59:14PM +0200, Javier González wrote:
>> As mentioned probably close to a dozen times over this thread and it's
>> predecessors: Keeping the per-file I/O hint API and mapping that to
>> FDP is fine. Exposing the overly-simplistic hints to the NVMe driver
>> through the entire I/O stack and locking us into that is not.
>
> I don't understand the "locking us into that" part.

The patches as submitted do the two following two things:

1) interpret the simple temperature hints to map to FDP reclaim handles
2) add a new interface to set the temperature hints per I/O

and also rely on an annoying existing implementation detail where the I/O
hints set on random files get automatically propagated to the block
device without file system involvement.

This means we can't easily make the nvme driver actually use smarter
hints that expose the actual FDP capabilities without breaking users
that relied on the existing behavior, especially the per-I/O hints that
counteract any kind of file system based data placement.