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sys var: consider global sort for tidb_ddl_reorg_max_write_speed #20334

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sys var: consider global sort for tidb_ddl_reorg_max_write_speed #20334

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sys var: consider GLOBAL SORT for tidb_ddl_reorg_max_write_speed
CbcWestwolf May 9, 2025
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CbcWestwolf May 9, 2025
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9 changes: 8 additions & 1 deletion system-variables.md
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Expand Up @@ -1552,12 +1552,19 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
- 类型：字符串
- 默认值：`0`
- 范围：`[0, 1PiB]`
- 这个变量用于限制每个 TiKV 节点写入的带宽，仅在开启添加索引加速功能时生效（由变量 [`tidb_ddl_enable_fast_reorg`](#tidb_ddl_enable_fast_reorg-从-v630-版本开始引入) 控制）。在数据量特别大的情况下（如数十亿行数据），降低加索引时写入 TiKV 节点的带宽可以有效减少对业务负载的影响。
- 这个变量用于限制索引回填过程中**单个 TiDB 节点向单个 TiKV 副本**写入的带宽，仅在开启添加索引加速功能时生效（由变量 [`tidb_ddl_enable_fast_reorg`](#tidb_ddl_enable_fast_reorg-从-v630-版本开始引入) 控制）。注意在开启[全局排序](/tidb-global-sort.md)时，多个 TiDB 节点可以同时向 TiKV 写入数据。在数据量特别大的情况下（如数十亿行数据），降低加索引时写入 TiKV 节点的带宽可以有效减少对业务负载的影响。
- 默认值 `0` 表示不限制写入带宽。
- 该变量可设置为带单位的格式或不带单位的格式。
- 当该变量值不带单位时，默认单位为字节每秒。例如 `67108864` 表示 `64MiB` 每秒。
- 当该变量值带单位时，支持的单位包括 KiB、MiB、GiB、TiB。例如，`'1GiB'` 表示 1 GiB 每秒， `'256MiB'` 表示 256 MiB 每秒。

示例：

假设当前有 4 个 TiDB 和 6 个 TiKV，每个 TiDB 均可以执行索引回填任务，Region 均匀分布在所有 TiKV 节点上，`tidb_ddl_reorg_max_write_speed` 被设置为 `100MiB`

* 当全局排序关闭时，同一时刻只有 1 个 TiDB 向 TiKV 写入，此时每个 TiKV 节点的最大写入带宽为 `100MiB`
* 当全局排序开启时，同一时刻所有 4 个 TiDB 都能向 TiKV 写入，此时每个 TiKV 节点的最大写入带宽为 `4 * 100MiB = 400MiB`

### `tidb_ddl_reorg_worker_cnt`

- 作用域：SESSION | GLOBAL
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