*: add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log #21009

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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ti-chi-bot · 2025-10-22T09:29:02Z

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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

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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

identify-slow-queries.md

zimulala · 2025-12-05T05:20:32Z

PTAL @lilin90 @huoyao1125

*: 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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zimulala requested review from XuHuaiyu and yibin87 November 19, 2025 02:36

zimulala force-pushed the zimuxia/slow-log branch from 3552306 to a405cf8 Compare November 19, 2025 03:12

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*: add test info and tiny update

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zimulala force-pushed the zimuxia/slow-log branch from a405cf8 to 437be1b Compare November 19, 2025 08:41

zimulala added 2 commits November 20, 2025 16:43

*: address a comment

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*: add version

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Update format

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zimulala added 2 commits December 5, 2025 12:12

*: address comments

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*: tiny update

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*: add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log #21009

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*: add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log #21009

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zimulala commented Dec 5, 2025

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zimulala commented Oct 17, 2025 •

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