*: 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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system-variables.md
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| - 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。 |
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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？
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| ### 相关系统变量 | ||
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| * [tidb_slow_log_threshold](/system-variables.md#tidb_slow_log_threshold)：设置慢日志的阈值，执行时间超过阈值的 SQL 语句将被记录到慢日志中。默认值是 300 ms。 | ||
| * [tidb_slow_log_rules](/system-variables.md#tidb_slow_log_rules)：用于定义慢日志的触发规则，支持多维度指标组合条件，实现更加灵活和精细化的日志记录控制。此变量在新版本中引入，逐步替代传统的单一阈值控制方式，即替代 `tidb_slow_log_threshold` 的使用。 |
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感觉要写一下，但不确定写 v9.0.0 还是 v9.0-beta3 @lilin90 知道吗？
identify-slow-queries.md
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| * [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。 |
| | └─IndexScan_16 cop 31281.857819905217 table:t, index:idx(a), range:[-inf,50000), keep order:false 25007 time:11ms, loops:25 N/A N/A | | ||
| +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ||
| ``` | ||
| ```sql |
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因为这个 sql 是上面 216 行的示例，所以我给它做了缩进。
system-variables.md
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| - 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。 |
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控制每个 TiDB 节点每秒，限定下是按照单节点算？
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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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