fdrd - Man Page

Flight Data Recorder daemon




The fdrd program is a daemon which enables ftrace probes, harvests ftrace data and (optionally) writes the data to a file.

The behavior of fdr is defined by configuration files stored in /etc/fdr.d. During service startup, fdr will process each file in the directory which has the suffix of .conf.  If new config files are added to the fdr.d directory, then the service must be restarted to recognize the new configuration information.

fdr is controlled by systemd (8) on systems where systemd is available.  Error messages from fdr can be viewed via systemctl, for example,

systemctl status -l fdr

Configuration File Syntax

The following keywords and options are recognized

instance iname [buffer-size]

Create a new ftrace instance called "iname".  This instance will appear in /sys/kernel/debug/tracing/instances

The optional buffer-size parameter can be used to control the size of the ftrace buffers for this instance in the kernel.  A suffix of 'k', 'K', 'm', 'M', 'g' or 'G' may be used to specify kilobytes, megabytes or gigabytes.

modprobe module-name

Force the named module to be loaded by fdr.  This can be useful when the module is normally loaded on demand and the probes cannot be enabled until the module is loaded.

enable subsystem-name/probe-name [filter]

Enable an ftrace probe in the specified subsystem.  Both the subsystem name and probe name are defined by the kernel.

The optional filter parameter allows an ftrace filter to be set as well.  This will limit the amount of data being emitted.  The syntax of the filter language is defined by ftrace itself and the parameters are defined by the static tracepoint being enabled in the kernel.

enable subsystem-name/all

Enable all ftrace probes for the subsystem.

disable subsystem-name/probe-name

Disable an ftrace probe in the specified subsystem.  This can be useful to disable selective probes when the "ALL" keyword has been used.

disable subsystem-name/all

Disable all probes in the specified subsystem.

saveto file-name [maxsize]

Save the output of enabled probes to the named file.  If the optional maxsize parameter is given, the daemon will initiate a log rotation, see Log Rotation below. A suffix of 'k', 'K', 'm', 'M', 'g' or 'G' may be used to specify kilobytes, megabytes or gigabytes.

If no saveto directive is present, then fdr will create the instance and enable the probes.  In this case, the data can be harvested manually by reading:


The ftrace buffers in the kernel are circular. If no process harvests the data, new data will overwrite old data.

minfree value

Limit the output by the daemon based on free space in the file system for the save file.  If free space percentage is below the specified value, no output will be written.

If no minfree directive is present, fdr will use 5% by default.

Log Rotation

fdr can use logrotate(8) to manage the output files.  By convention, /etc/logrotate.d/instance-name controls the behavior of logrotate.

fdr will also invoke logrotate directly at startup and when reaching the maxsize limit for the save file.

See Also




Bill Baker and Calum Mackay


15 June 2021