Re: [PATCH v2] f2fs: change write_hint for hot/warm nodes

[Chao Yu]

On 2020/11/4 2:58, Jaegeuk Kim wrote:
> On 11/03, Daejun Park wrote:
> > In the fs-based mode of F2FS, the mapping of hot/warm node to
> > WRITE_LIFE_NOT_SET should be changed to WRITE_LIFE_SHORT.
> >
> > As a result of analyzing the write pattern of f2fs using real workload,
> > hot/warm nodes have high update ratio close to hot data.[*]
> > However, F2FS passes write hints for hot/warm nodes as WRITE_LIFE_NOT_SET.
>
> I prefer to keep it as is, since basically node blocks should be separatly
> stored from data blocks in different erase blocks in order to match F2FS GC

Correct, I missed that the change will break HOT_DATA type and {HOT,WARM}_NODE
type data separation in FTL.

> and FTL GC units. And, we don't do IPU for node blocks which doesn't make sense
> to say *update ratio*.

IMO, if fs discard is fast enough, both IPU and OPU will cause original blkaddr in
FTL being invalid, we can compare update ratio on {I,O}PU IOs inside FTL; otherwise,
we can't.

Thanks,

> > Furthermore, WRITE_LIFE_NOT_SET is a default value of write hint when it is
> > not provided from the file system.
> > In storage, write_hint is used to distinguish streams (e.g. NVMe).
> > So, the hot/warm node of F2FS is not distinguished from other write_hints,
> > which can make the wrong stream seperation.
> >
> > * Liang, Yu, et al. "An empirical study of F2FS on mobile devices." 2017
> > IEEE 23rd International Conference on Embedded and Real-Time Computing
> > Systems and Applications (RTCSA).
> >
> > Signed-off-by: Daejun Park <daejun7.park@xxxxxxxxxxx>
> > ---
> > v2: update documentation and comments
> > ---
> >   Documentation/filesystems/f2fs.rst | 4 ++--
> >   fs/f2fs/segment.c                  | 6 +++---
> >   2 files changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
> > index b8ee761c9922..afa3da7cfade 100644
> > --- a/Documentation/filesystems/f2fs.rst
> > +++ b/Documentation/filesystems/f2fs.rst
> > @@ -717,8 +717,8 @@ WRITE_LIFE_LONG       "                        WRITE_LIFE_LONG
> >   ===================== ======================== ===================
> >   User                  F2FS                     Block
> >   ===================== ======================== ===================
> > -                      META                     WRITE_LIFE_MEDIUM;
> > -                      HOT_NODE                 WRITE_LIFE_NOT_SET
> > +                      META                     WRITE_LIFE_MEDIUM
> > +                      HOT_NODE                 WRITE_LIFE_SHORT
> >                         WARM_NODE                "
> >                         COLD_NODE                WRITE_LIFE_NONE
> >   ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 1596502f7375..577ab7516c6b 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -3160,8 +3160,8 @@ int f2fs_rw_hint_to_seg_type(enum rw_hint hint)
> >    *
> >    * User                  F2FS                     Block
> >    * ----                  ----                     -----
> > - *                       META                     WRITE_LIFE_MEDIUM;
> > - *                       HOT_NODE                 WRITE_LIFE_NOT_SET
> > + *                       META                     WRITE_LIFE_MEDIUM
> > + *                       HOT_NODE                 WRITE_LIFE_SHORT
> >    *                       WARM_NODE                "
> >    *                       COLD_NODE                WRITE_LIFE_NONE
> >    * ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
> > @@ -3208,7 +3208,7 @@ enum rw_hint f2fs_io_type_to_rw_hint(struct f2fs_sb_info *sbi,
> >   				return WRITE_LIFE_EXTREME;
> >   		} else if (type == NODE) {
> >   			if (temp == WARM || temp == HOT)
> > -				return WRITE_LIFE_NOT_SET;
> > +				return WRITE_LIFE_SHORT;
> >   			else if (temp == COLD)
> >   				return WRITE_LIFE_NONE;
> >   		} else if (type == META) {
> > --
> > 2.25.1
> .