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tidb: add description for var tidb_ddl_reorg_max_write_speed #19071

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tidb: add description for var tidb_ddl_reorg_max_write_speed #19071

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tidb: add description for var `tidb_ddl_reorg_max_write_speed`
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11 changes: 11 additions & 0 deletions system-variables.md
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Expand Up @@ -1516,6 +1516,17 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
- 可选值：`PRIORITY_LOW`、`PRIORITY_NORMAL`、`PRIORITY_HIGH`
- 这个变量用来设置 `ADD INDEX` 操作 `re-organize` 阶段的执行优先级，可设置为 `PRIORITY_LOW`/`PRIORITY_NORMAL`/`PRIORITY_HIGH`。

### `tidb_ddl_reorg_max_write_speed` <span class="version-mark">从 v8.5.0 版本开始引入</span>
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- 作用域：GLOBAL
- 是否持久化到集群：是
- 是否受 Hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value) 控制：否
- 类型：整数型
- 默认值：`0`
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- 范围：`[0, 1125899906842624]`（即最大可设置为 1 PiB）
- 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，降低加索引时写入 TiKV 节点的带宽可以有效减少对前端负载的影响。
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- 默认值 `0` 表示不限制写入带宽。默认单位为字节每秒，也可以通过 `'1GiB'`、`256MiB` 等格式设置该变量。
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### `tidb_ddl_reorg_worker_cnt`

- 作用域：SESSION | GLOBAL
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