release note about change to log file flushing

ros2/rcl_logging#95

source/Releases/Release-Iron-Irwini.rst

@@ -68,6 +68,20 @@ Classes that were changed:
 
 The old class names are still there, but will be deprecated.
 
+Change to the default console logging file flushing behavior
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This specifically applies to the default ``spdlog`` based logging backend in ROS 2, implemented in ``rcl_logging_spdlog``, and log file flushing was changed to flush every time an "error" log message is used, e.g. each `RCLCPP_ERROR()` call, and also periodically every five seconds.
+
+Previously, ``spdlog`` was used without configuring anything other than creating the sink for logging to a file.
+
+We tested the change and did not find that the CPU overhead was significant, even on machines with slow disks (e.g. sd cards).
+However, if this change is causing you problems you can get the old behavior by setting the ``RCL_LOGGING_SPDLOG_EXPERIMENTAL_OLD_FLUSHING_BEHAVIOR=1`` environment variable.
+
+Later we would like to have support for a full configuration file, giving you more flexibility in how the logging is done, but that is work that is only planned right now.
+
+See this issue for more details: https://github.com/ros2/rcl_logging/pull/95
+
 Known Issues
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