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

[Jens Axboe <axboe@kernel.dk>]

On 10/11/24 3:02 AM, Christoph Hellwig 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.
> 

I think that last argument is a straw man - for any kind of interface
like this, we've ALWAYS just had the rule that any per-whatever
overrides the generic setting. Per IO hints would do the same thing. Is
this a mess if a user has assigned a file hint and uses other per IO
hints on that very same file and they differ? Certainly, but that's
really just the user/app being dumb. Or maybe being smart, as this
particular one block is always hot in the file (yeah contrived case).

The point is, if you mix and match per-io hints and per-file hints, then
you're probably being pretty silly and nobody should do that. I don't
think this is a practical concern at all.

-- 
Jens Axboe