initial commit, migrating from draft status

Async-Write.md

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+### Async Writes
+
+The number of concurrent operations issued for the async write I/O class
+follows a piece-wise linear function defined by a few adjustable points.
+
+```
+       |              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
+```
+
+Until the amount of dirty data exceeds a minimum percentage of the dirty
+data allowed in the pool, the I/O scheduler will limit the number of
+concurrent operations to the minimum. As that threshold is crossed, the
+number of concurrent operations issued increases linearly to the maximum at
+the specified maximum percentage of the dirty data allowed in the pool.
+
+Ideally, the amount of dirty data on a busy pool will stay in the sloped
+part of the function between zfs_vdev_async_write_active_min_dirty_percent
+and zfs_vdev_async_write_active_max_dirty_percent. If it exceeds the
+maximum percentage, this indicates that the rate of incoming data is
+greater than the rate that the backend storage can handle. In this case, we
+must further throttle incoming writes, as described in the next section.