- * Dispatch the parent zio in its own taskq so that
- * the child can continue to make progress. This also
- * prevents overflowing the stack when we have deeply nested
- * parent-child relationships.
+ * If we can tell the caller to execute this parent next, do
+ * so. Otherwise dispatch the parent zio as its own task.
+ *
+ * Having the caller execute the parent when possible reduces
+ * locking on the zio taskq's, reduces context switch
+ * overhead, and has no recursion penalty. Note that one
+ * read from disk typically causes at least 3 zio's: a
+ * zio_null(), the logical zio_read(), and then a physical
+ * zio. When the physical ZIO completes, we are able to call
+ * zio_done() on all 3 of these zio's from one invocation of
+ * zio_execute() by returning the parent back to
+ * zio_execute(). Since the parent isn't executed until this
+ * thread returns back to zio_execute(), the caller should do
+ * so promptly.
+ *
+ * In other cases, dispatching the parent prevents
+ * overflowing the stack when we have deeply nested
+ * parent-child relationships, as we do with the "mega zio"
+ * of writes for spa_sync(), and the chain of ZIL blocks.