[PATCH] nvme: enable FDP support

[Kanchan Joshi]

On Tue, May 14, 2024 at 2:40 PM Viacheslav Dubeyko <slava at dubeyko.com> wrote:
>
>
>
> > 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?

As many as already defined (in rw_hint.h). Possible to go higher too.
No hard limitation per se. Write is not going to fail even if it sends
a hint that does not exist.

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

It does not need to detect. File system sees write-hints; FDP is a
lower-level detail.