ffx log
is a log-viewing utility built into ffx
. This guide describes how to
configure and use ffx log
to view logs on your Fuchsia device.
To start the log viewer, run the following command:
ffx log
This command prints the current log contents and leaves the connection open to
stream new log entries. To print the current contents of the log and exit, use dump
:
ffx log dump
Proactive logging
The ffx
daemon persists in the background after an ffx
command is run. The daemon proactively
discovers Fuchsia devices and connects to them as they become reachable.
With proactive logging, the ffx
daemon begins reading logs from a target device in the background
as soon as it's connected. The logs are cached on the host machine, up to a configured space limit.
When the space limit is reached, logs are "rotated": the oldest logs are deleted to make room for
the newest ones.
This means that when you view logs using ffx log
, logs are actually read from the cache on the
host machine, not directly from the target device. In general, this should not add any noticeable
delay to the log viewer, except in rare cases where the device is producing extremely large log
volumes.
Features
Since logs are cached on the host machine, you can view logs that have been cached from a target device that are from previous boots of the device. For example, if a device crashes, you might be able to view the logs from the time just before the crash if they were cached in time.
You can use ffx log dump
to view logs from a previous session. For example, to view logs from your
device's previous boot:
ffx --target <NODENAME> log dump ~1
~1
identifies the session relative to the latest one you want to view, where 0
is reserved for
the currently active session for that target device (whether or not a currently active session exists). You
can view earlier boots by using ~2
, ~3
, and so on.
Configuration
There are 3 configuration settings relevant to the proactive log cache:
proactive_log.max_sessions_per_target
: The maximum number of boot sessions to keep cached on the host. Default is 5 (that is, after 6 reboots, the logs from the oldest boot session are deleted).proactive_log.max_session_size_bytes
: The maximum number of bytes to be cached for each session. Default is 100MB (that is, after 100MB of logs are on-disk, the oldest chunk of logs for that session are deleted)proactive_log.max_log_size_bytes
: The maximum number of bytes to be used in a single log chunk. You should not generally need to change this setting. Default is 1MB.
Symbolization
Logs are symbolized in the background as they are read from the device (before they are written to
the host log cache). However, this background processing means that misconfigurations in the
symbolizer
host tool or with the symbol index can cause logs to be not symbolized without any
visible warning. Errors encountered when setting up the symbolizer
tool are logged to the ffx
daemon log.
Users working with the Fuchsia source checkout setup do not need to perform any extra configuration;
symbolization takes place automatically as in fx log
. Users working without the Fuchsia source
checkout setup need to configure the symbol index appropriate to their development environment.
The ffx log
command tries to detect common misconfigurations in the symbolizer
tool, but cannot
detect all of them. If your logs are not being symbolized, please
file a bug.
Configuring symbolizer
There are two configuration parameters relevant to symbolization:
proactive_log.symbolize.enabled
: Toggles whether symbolization is attempted. Default istrue
.proactive_log.symbolize.extra_args
: A raw string of additional parameters passed directly to thesymbolizer
host tool. This can be used to, for example, configure remote symbol servers. Default is""
.
Filtering logs
The ffx log
command provides additional options to filter the logs captured from the target
device. You can apply filters to the log based on timestamps, component, tags, or log level.
ffx log --filter hello-world --severity error
For a complete list of filtering options, run ffx log --help
.
Log settings
Log filters modify how the captured logs are displayed by ffx log
, but they do not affect the
log entries emitted by components on the target device. Use the --set-severity
option to send a
request to configure the log settings of specific components during the
logging session. This adjusts the log level applied to any component matching the provided
component selector for recording logs.
ffx log --set-severity component-selector#log-level
You can use this to temporarily enable logs that are below the minimum severity configure by your
component, such as DEBUG
or TRACE
logs, or to suppress noisy logs from a component to improve
performance.
The following example enables debug logs for the core/audio
component, and suppresses all log
messages except errors from networking components:
$ ffx log --set-severity core/audio#DEBUG --set-severity core/network/**#ERROR