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Logging

This describes the new logger, SYS_LOGGER set to 1.

The logger is able to log any ORB topic with all included fields. Everything
necessary is generated from the .msg file, so that only the topic name needs
to be specified. An optional interval parameter specifies the maximum logging
rate of a certain topic. All existing instances of a topic are logged.

The output log format is ULog.

Usage

By default, logging is automatically started when arming, and stopped when
disarming. A new log file is created for each arming session on the SD card. To
display the current state, use logger status on the console. If you want to
start logging immediately, use logger on. This overrides the arming state, as
if the system was armed. logger off undoes this.

Use

logger help

for a list of all supported logger commands and parameters.

Configuration

The list of logged topics can be customized with a file on the SD card. Create a
file etc/logging/logger_topics.txt on the card with a list of topics:

<topic_name>, <interval>

The <interval> is optional, and if specified, defines the minimum interval in
ms between two logged messages of this topic. If not specified, the topic is
logged at full rate.

The topics in this file replace all of the default logged topics.

Scripts

There are several scripts to analyze and convert logging files in the
pyulog repository.

Dropouts

Logging dropouts are undesired and there are a few factors that influence the
amount of dropouts:

• Most SD cards we tested exhibit multiple pauses per minute. This shows
  itself as a several 100 ms delay during a write command. It causes a dropout
  if the write buffer fills up during this time. This effect depends on the SD
  card (see below).
• Formatting an SD card can help to prevent dropouts.
• Increasing the log buffer helps.
• Decrease the logging rate of selected topics or remove unneeded topics from
  being logged (info.py <file> is useful for this).

SD Cards

The following provides performance results for different SD cards.
Tests were done on a Pixracer; the results are applicable to Pixhawk as well.

More important than the mean write speed is the maximum write time per block (of
4 KB). This defines the minimum buffer size: the larger this maximum, the larger
the log buffer needs to be to avoid dropouts. Logging bandwidth with the default
topics is around 50 KB/s, which all of the SD cards satisfy.

By far the best card we know so far is the SanDisk Extreme U3 32GB. This
card is recommended, because it does not exhibit write time spikes (and thus
virtually no dropouts). Different card sizes might work equally well, but the
performance is usually different.

You can test your own SD card with sd_bench -r 50, and report the results to
https://github.com/PX4/Firmware/issues/4634.

Log Streaming

The traditional and still fully supported way to do logging is using an SD card
on the FMU. However there is an alternative, log streaming, which sends the
same logging data via MAVLink. This method can be used for example in cases
where the FMU does not have an SD card slot (e.g. Intel® Aero Ready to Fly Drone) or simply to avoid
having to deal with SD cards. Both methods can be used independently and at the
same time.

The requirement is that the link provides at least ~50KB/s, so for example a
WiFi link. And only one client can request log streaming at the same time. The
connection does not need to be reliable, the protocol is designed to handle
drops.

There are different clients that support ulog streaming:

• mavlink_ulog_streaming.py script in Firmware/Tools.
• QGroundControl:
  
• MAVGCL

Diagnostics

• If log streaming does not start, make sure the logger is running (see
  above), and inspect the console output while starting.
• If it still does not work, make sure that Mavlink 2 is used. Enforce it by
  setting MAV_PROTO_VER to 2.
• Log streaming uses a maximum of 70% of the configured mavlink rate (-r
  parameter). If more is needed, messages are dropped. The currently used
  percentage can be inspected with mavlink status (1.8% is used in this
  example):

  instance #0:
        GCS heartbeat:  160955 us ago
        mavlink chan: #0
        type:           GENERIC LINK OR RADIO
        flow control:   OFF
        rates:
        tx: 95.781 kB/s
        txerr: 0.000 kB/s
        rx: 0.021 kB/s
        rate mult: 1.000
        ULog rate: 1.8% of max 70.0%
        accepting commands: YES
        MAVLink version: 2
        transport protocol: UDP (14556)

  Also make sure txerr stays at 0. If this goes up, either the NuttX sending
  buffer is too small, the physical link is saturated or the hardware is too
  slow to handle the data.