On Tue, May 14, 2024 at 2:40 PM Viacheslav Dubeyko <slava@xxxxxxxxxxx> wrote: > > > > > On May 15, 2024, at 6:30 AM, Kanchan Joshi <joshiiitr@xxxxxxxxx> wrote: > > > > On Tue, May 14, 2024 at 1:00 PM Viacheslav Dubeyko <slava@xxxxxxxxxxx> wrote: > >>> On May 14, 2024, at 9:47 PM, Kanchan Joshi <joshiiitr@xxxxxxxxx> wrote: > >>> > >>> On Mon, May 13, 2024 at 2:04 AM Viacheslav Dubeyko <slava@xxxxxxxxxxx> wrote: > >>>> > >>>> > >>>> > >>>>> On May 10, 2024, at 4:40 PM, Kanchan Joshi <joshi.k@xxxxxxxxxxx> 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.
