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Subject: [PATCH v2] f2fs: change write_hint for hot/warm nodes
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Sender: Daejun Park <daejun7.park@samsung.com>
From:   Daejun Park <daejun7.park@samsung.com>
To:     "jaegeuk@kernel.org" <jaegeuk@kernel.org>,
        "chao@kernel.org" <chao@kernel.org>,
        Daejun Park <daejun7.park@samsung.com>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-f2fs-devel@lists.sourceforge.net" 
        <linux-f2fs-devel@lists.sourceforge.net>,
        Chao Yu <yuchao0@huawei.com>,
        yongmyung lee <ymhungry.lee@samsung.com>,
        Jieon Seol <jieon.seol@samsung.com>,
        Sang-yoon Oh <sangyoon.oh@samsung.com>,
        Mankyu PARK <manq.park@samsung.com>,
        Sung-Jun Park <sungjun07.park@samsung.com>,
        Keoseong Park <keosung.park@samsung.com>,
        SEUNGUK SHIN <seunguk.shin@samsung.com>,
        Jinyoung CHOI <j-young.choi@samsung.com>,
        Jaemyung Lee <jaemyung.lee@samsung.com>
Message-ID: <20201103083832epcms2p6c8b0e4470f1392772317ab2b25256b3d@epcms2p6>
Date:   Tue, 03 Nov 2020 17:38:32 +0900
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References: <CGME20201103083832epcms2p6c8b0e4470f1392772317ab2b25256b3d@epcms2p6>
Precedence: bulk

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.

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@samsung.com>
---
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