*: add slow log control variables (`tidb_slow_log_rules`, `tidb_slow_log_max_per_sec`) and hint `WRITE_SLOW_LOG` by zimulala · Pull Request #21009 · pingcap/docs-cn

zimulala · 2025-10-17T07:46:22Z

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pingcap/tidb#64010
Add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log

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Other reference link(s): Slow Log Enhancement: Multi-Dimensional Triggering and Dynamic Scope Control tidb#62959

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zimulala · 2025-11-04T08:54:19Z

system-variables.md

+- 默认值：`0`
+- 类型：整数型
+- 范围：`[0, 1000000]`
+- 控制每秒打印慢日志每秒的次数上限，默认值 0。当该变量值为 0 时，则表示不限制每秒打印的慢日志数量。当该变量值大于 0 时，TiDB 会限制每秒打印的慢日志数量不超过该值，超过部分的慢日志将被丢弃，不会写入慢日志文件。该变量常与 [`tidb_slow_log_rules`](#tidb_slow_log_rules) 结合使用，以防止在高负载情况下产生过多的慢日志。


其中峰值可能超过配值（峰值 = 2*tidb_slow_log_max_per_sec），平均打印速率仍然受 tidb_slow_log_max_per_sec 限制。这个内容是否需要说明？

identify-slow-queries.md

yibin87 · 2025-11-19T08:30:55Z

identify-slow-queries.md

+* [tidb_slow_log_max_per_sec](/system-variables.md#tidb_slow_log_max_per_sec)：设置控制每秒打印慢日志的上限，默认值为 0。
+  * 当值为 0，其表示不限制每秒打印的慢日志数量。
+  * 当值大于 0 时，TiDB 每秒最多打印指定数量的慢日志，超过部分将被丢弃，不会写入慢日志文件。
+  * 建议在启用了 tidb_slow_log_rules 后配置该变量，以防规则触发频繁打印慢日志。


要不要再补充下，可能影响系统查询性能。

在下面几行的建议里面

identify-slow-queries.md

system-variables.md

yibin87

LGTM

ti-chi-bot · 2025-11-26T01:07:38Z

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zimulala · 2025-11-26T01:42:30Z

PTAL @lilin90

zimulala · 2025-12-01T11:09:28Z

PTAL @lilin90 @huoyao1125

lilin90

/bot-review

Copilot

Pull request overview

This PR adds documentation for new slow log control features in TiDB v9.0.0, including two system variables (tidb_slow_log_rules and tidb_slow_log_max_per_sec) and a new hint (WRITE_SLOW_LOG). These features provide more flexible and fine-grained control over slow query logging compared to the traditional single threshold approach.

Key Changes:

Introduced tidb_slow_log_rules variable to define multi-dimensional slow log trigger rules
Added tidb_slow_log_max_per_sec variable to rate-limit slow log output per TiDB instance
Documented the WRITE_SLOW_LOG hint to force slow log output regardless of thresholds

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
system-variables.md	Added documentation for two new system variables: `tidb_slow_log_rules` (SESSION/GLOBAL scope for defining trigger rules) and `tidb_slow_log_max_per_sec` (GLOBAL scope for rate limiting)
identify-slow-queries.md	Added documentation for the `WRITE_SLOW_LOG` hint and expanded the system variables section with detailed explanations of the new variables, including usage scenarios and interaction between rules and thresholds

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identify-slow-queries.md

system-variables.md

zimulala · 2026-01-19T11:09:41Z

@lilin90
PTAL

identify-slow-queries.md

system-variables.md

identify-slow-queries.md

Co-authored-by: Lilian Lee <lilin@pingcap.com>

system-variables.md

lilin90

/lgtm

ti-chi-bot · 2026-01-21T02:06:46Z

[LGTM Timeline notifier]

Timeline:

2026-01-20 09:30:49.406129149 +0000 UTC m=+486277.020086005: ☑️ agreed by lilin90.
2026-01-21 02:06:45.182123954 +0000 UTC m=+546032.796080820: ☑️ agreed by hfxsd.

ti-chi-bot · 2026-01-21T02:41:36Z

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lilin90

/approve

hfxsd · 2026-01-21T02:53:10Z

/approve

ti-chi-bot · 2026-01-21T02:53:18Z

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd, lilin90

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ti-chi-bot · 2026-03-02T09:43:44Z

In response to a cherrypick label: new pull request created to branch release-8.5: #21406.

*: add two variables tidb_slow_log_rules and tidb_slow_log_max_per_sec

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ti-chi-bot bot added missing-translation-status This PR does not have translation status info. size/M Denotes a PR that changes 30-99 lines, ignoring generated files. labels Oct 17, 2025

qiancai added v9.0-beta.3 This PR/issue applies to TiDB v9.0-beta.3. translation/doing This PR’s assignee is translating this PR. and removed missing-translation-status This PR does not have translation status info. labels Oct 17, 2025

qiancai assigned lilin90 Oct 21, 2025

*: add WRITE_SLOW_LOG hint

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zimulala changed the title *: add two variables tidb_slow_log_rules and tidb_slow_log_max_per_sec *: add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log Oct 22, 2025

zimulala commented Nov 4, 2025

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*: add test info and tiny update

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zimulala added 2 commits November 20, 2025 16:43

*: address a comment

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*: add version

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ti-chi-bot bot added size/L Denotes a PR that changes 100-499 lines, ignoring generated files. and removed size/M Denotes a PR that changes 30-99 lines, ignoring generated files. labels Dec 4, 2025

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zimulala and others added 7 commits January 20, 2026 16:44

Update identify-slow-queries.md

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Co-authored-by: Lilian Lee <lilin@pingcap.com>

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Co-authored-by: Lilian Lee <lilin@pingcap.com>

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ti-chi-bot bot merged commit 1538836 into pingcap:master Jan 21, 2026
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zimulala deleted the zimuxia/slow-log branch January 21, 2026 02:55

XuHuaiyu mentioned this pull request Feb 5, 2026

完善 tidb_slow_log_rules 文档：字段/单位/取值范围（min-max）与语法约束 #21324

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lilin90 added the needs-cherry-pick-release-8.5 Should cherry pick this PR to release-8.5 branch. label Mar 2, 2026

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zimulala commented Oct 17, 2025 •

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