[PATCH] nvme: enable FDP support

[Viacheslav Dubeyko]

> On May 15, 2024, at 6:30 AM, Kanchan Joshi <joshiiitr at gmail.com> wrote:
> 
> On Tue, May 14, 2024 at 1:00 PM Viacheslav Dubeyko <slava at dubeyko.com> wrote:
>>> On May 14, 2024, at 9:47 PM, Kanchan Joshi <joshiiitr at gmail.com> wrote:
>>> 
>>> On Mon, May 13, 2024 at 2:04 AM Viacheslav Dubeyko <slava at dubeyko.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On May 10, 2024, at 4:40 PM, Kanchan Joshi <joshi.k at samsung.com> wrote:
>>>>> 
>>>>> Flexible Data Placement (FDP), as ratified in TP 4146a, allows the host
>>>>> to control the placement of logical blocks so as to reduce the SSD WAF.
>>>>> 
>>>>> Userspace can send the data lifetime information using the write hints.
>>>>> The SCSI driver (sd) can already pass this information to the SCSI
>>>>> devices. This patch does the same for NVMe.
>>>>> 
>>>>> Fetches the placement-identifiers (plids) if the device supports FDP.
>>>>> And map the incoming write-hints to plids.
>>>>> 
>>>> 
>>>> 
>>>> Great! Thanks for sharing  the patch.
>>>> 
>>>> Do  we have documentation that explains how, for example, kernel-space
>>>> file system can work with block layer to employ FDP?
>>> 
>>> This is primarily for user driven/exposed hints. For file system
>>> driven hints, the scheme is really file system specific and therefore,
>>> will vary from one to another.
>>> F2FS is one (and only at the moment) example. Its 'fs-based' policy
>>> can act as a reference for one way to go about it.
>> 
>> Yes, I completely see the point. I would like to employ the FDP in my
>> kernel-space file system (SSDFS). And I have a vision how I can do it.
>> But I simply would like to see some documentation with the explanation of
>> API and limitations of FDP for the case of kernel-space file systems.
> 
> Nothing complicated for early experimentation.
> Once FS has determined the hint value, it can put that into
> bio->bi_write_hint and send bio down.
> 
> If SSDFS cares about user-exposed hints too, it can choose different
> hint values than what is exposed to user-space.
> Or it can do what F2FS does - use the mount option as a toggle to
> reuse the same values either for user-hints or fs-defined hints.

How many hint values file system can use? Any limitations here?

And how file system can detect that it’s FDP-based device?

Thanks,
Slava.