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

From: Chao Yu
Date: Wed Nov 04 2020 - 22:16:04 EST


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
.