# Async Write I/O 调度

为 async write I/O 类发出的并发操作数量遵循一个分段线性函数，该函数由若干可调节点定义：

```
|                   o---------| <-- zfs_vdev_async_write_max_active
  ^    |                  /^         |
  |    |                 / |         |
active |                /  |         |
 I/O   |               /   |         |
count  |              /    |         |
       |             /     |         |
       |------------o      |         | <-- zfs_vdev_async_write_min_active
      0|____________^______|_________|
       0%           |      |        100% of zfs_dirty_data_max
                    |      |
                    |      `-- zfs_vdev_async_write_active_max_dirty_percent
                    `--------- zfs_vdev_async_write_active_min_dirty_percent
```

在脏数据量未超过存储池允许的脏数据最小百分比之前，I/O 调度器会将并发操作数量限制为最小值。当超过该阈值后，发出的并发操作数量会随脏数据量线性增加，并在达到所指定的允许的脏数据最大百分比时达到最大值。

理想情况下，繁忙存储池中的脏数据量应保持在 `zfs_vdev_async_write_active_min_dirty_percent` 与 `zfs_vdev_async_write_active_max_dirty_percent` 之间的函数斜坡区间内。如果超过最大百分比，则表明传入数据速率大于后端存储能够处理的速率。在这种情况下，必须进一步对传入写入进行节流。详见 ZIO Transaction Delay。

参考代码： [vdev\_queue.c](https://github.com/openzfs/zfs/blob/master/module/zfs/vdev_queue.c#L42)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://book.bsdcn.org/zfs/xing-neng-tiao-you/async-write.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.