A virtual machine instance has four virtual serial ports. The instance's operating system, BIOS, and other system-level entities often write output to the serial ports, which makes them useful for troubleshooting crashes, failed boots, startup issues, or shutdown issues.
This page describes methods to view serial port output, including using Stackdriver logging to retain serial port output even after an instance is stopped or deleted. If you need to send commands to a serial port while an instance is running, see Interacting with the Serial Console.
Serial port output is accessible through the GCP Console,
tool, and API, only when the VM instance is running, and logs are limited to the
most recent 1 MB of output per port.
Before you begin
- If you want to use the command-line examples in this guide:
- If you want to use the API examples in this guide, set up API access.
- If you want to log serial port output in Stackdriver, familiarize yourself with Stackdriver Logging.
Enabling and disabling serial port output logging to Stackdriver
You can control whether your instances send serial port output to Stackdriver by setting project- or instance-level metadata. You can also disable the feature for all of the users in your organization by setting an organization policy.
Setting project and instance metadata
If serial port output logging to Stackdriver is not
constrained for your organization,
then you can enable or disable it for projects and for individual VM
instances setting the metadata
At the project level,
serial-port-logging-enable is set to
true, then all VM instances in the
project inherit that setting implicitly. If it is set to
at the VM instance level,
then it is enabled for that VM only, regardless of the project setting.
Conversely, to disable serial port output logging for projects or instances, set
serial-port-logging-enable metadata value to
false. When this metadata
is set to
false at the project level, serial port output logging to
Stackdriver is disabled for all instances in the project. When it is set to
false at the instance level, then serial port logging is disabled for that VM
only, regardless of the project setting.
You can set metadata through the Google Cloud Platform Console, the
gcloud tool, or
the API. See Setting custom metadata
For example, you can use the following
gcloud command to enable serial port
output logging to Stackdriver for an existing instance. The command will fail
if an organizational policy prevents this configuration.
gcloud compute instances add-metadata [INSTANCE_NAME] \ --metadata serial-port-logging-enable=true
To disable serial port output logging to Stackdriver, set
gcloud compute instances add-metadata [INSTANCE_NAME] \ --metadata serial-port-logging-enable=false
From within Stackdriver, you can
create an exclusion filter to
remove specific serial port entries from the Logs Viewer. For example, with
serial-port-logging-enable=true at the project level, you can disable
serial port outuput logging for specific VM instances with an advanced
filter like the following one.
logName = "projects/google.com:serial-output-logging-demo/logs/serialconsole.googleapis.com%2Fserial_port_1_output" resource.type = "gce_instance" resource.labels.instance_id != "[INSTANCE_1_ID]" resource.labels.instance_id != "[INSTANCE_2_ID]"
Setting an organization policy
You can disable serial port output logging to Stackdriver for your entire
organization by setting an
Organization Policy, which
constrains certain configurations of GCP resources. Specifically,
set the following boolean constraint:
Creating and managing organization policies for more information.
Disabling serial port logging by setting
true is not
retroactive: existing VM instances that have metadata to enable serial port
logging to Stackdriver will continue to log to Stackdriver unless you reset
the metadata for those instances.
After setting this organization constraint to
true, you cannot set instance or
project metadata to enable serial port output logging to Stackdriver for any
instances within the organization.
Viewing serial port output
gcloud instances get-serial-port-output
gcloud compute instances get-serial-port-output [INSTANCE_NAME] \ --port [PORT] \ --start [START] \ --zone [ZONE]
[INSTANCE_NAME]is the name of the instance.
[PORT]is number of the port (
4) for which you want to view output. System-level entities typically use the first serial port (port 1), which is also known as the serial console. By default, the output of the first serial port is returned.
[START]specifies the byte index (zero-based) of the first byte you want returned. Use this flag if you want to continue getting the output from a previous request that was too long to return in one attempt.
In the API, create a
get request to the
- Enable serial port logging to Stackdriver
- Go to the VM instances page.
- Select the VM instance for which you want to view startup agent logs.
Under Logs, click Stackdriver Logging to view Stackdriver logs.
Expand the All logs dropdown menu and select the serial port output that you want to see. System-level entities typically use the first serial port (port 1), which is also known as the serial console. If a port does not appear in the dropdown menu, it has no available output.
Optionally, expand the Any log level dropdown menu to mute logs that are below the severity level that you want to see. For example, if you select the "Info" log level, you will mute "Debug" log entries. "Debug" entries are typically used only by the BIOS at boot time.
See the Stackdriver documentation on Viewing logs for more details, including basic and advance filtering.
Handling non-UTF8 characters
Serial port output is escaped using the open source
Abseil C++ library's
method, so non-UTF8 characters are encoded as hex strings. You can use the
method to get the exact output that was emitted to the serial port.