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docs/Async-Write.rst

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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.