How to troubleshoot failing instances

If your instance fails to start and ends up in an error state, this usually indicates a bigger issue related to either the image that you used to create the instance or the server configuration.

To troubleshoot the problem, complete the following steps:

1. Save the relevant log files and debug information:

   Instance log

   Display the instance log:

   CLI
   lxc info <instance_name> --show-log

   API
   lxc query --request GET /1.0/instances/<instance_name>/logs/lxc.log

   UI
   Navigate to the instance detail page and switch to the Logs tab to view the available log files.

   Console log

   Display the console log:

   CLI
   lxc console <instance_name> --show-log
   This command is available only for containers.

   API
   lxc query --request GET /1.0/instances/<instance_name>/console
   This endpoint is available only for containers.

   UI
   Navigate to the instance detail page and switch to the Console tab to view the console. The console is displayed only when the instance is running.

   Detailed server information

   The LXD snap includes a tool that collects the relevant server information for debugging. Enter the following command to run it:

   sudo lxd.buginfo

2. Reboot the machine that runs your LXD server.

3. Try starting your instance again. If the error occurs again, compare the logs to check if it is the same error.

   If it is, and if you cannot figure out the source of the error from the log information, open a question in the forum (opens in a new tab). Make sure to include the log files you collected.

Troubleshooting examples

See the following sections for some typical methods of troubleshooting an instance.

Debug systemd init

Here is how to enable systemd debug level messages (opens in a new tab) for the c1 container:

lxc config set c1 raw.lxc 'lxc.init.cmd = /sbin/init systemd.log_level=debug'
lxc start c1

Now that the container has started, you can check for the debug messages in the journal:

lxc exec c1 -- journalctl

Emergency systemd shell

Here is how to get an emergency shell on an instance using systemd:

lxc config set c1 raw.lxc 'lxc.init.cmd = /sbin/init emergency'
lxc start c1

Now that the container has started, you can enter the emergency shell using the console (hit the Enter key once in):

lxc console c1

Issue starting RHEL 7 container

In this example, let's investigate a RHEL 7 system in which systemd cannot start.

~$ lxc console --show-log rhel7
Console log:
Failed to insert module 'autofs4'
Failed to insert module 'unix'
Failed to mount sysfs at /sys: Operation not permitted
Failed to mount proc at /proc: Operation not permitted
[!!!!!!] Failed to mount API filesystems, freezing.

The errors here say that /sys and /proc cannot be mounted - which is correct in an unprivileged container. However, LXD mounts these file systems automatically if it can.

The container requirements specify that every container must come with an empty /dev, /proc and /sys directory, and that /sbin/init must exist. If those directories don't exist, LXD cannot mount them, and systemd will then try to do so. As this is an unprivileged container, systemd does not have the ability to do this, and it then freezes.

So you can see the environment before anything is changed, and you can explicitly change the init system in a container using the raw.lxc configuration parameter. This is equivalent to setting init=/bin/bash on the Linux kernel command line.

lxc config set rhel7 raw.lxc 'lxc.init.cmd = /bin/bash'

Here is what it looks like:

~$ lxc config set rhel7 raw.lxc 'lxc.init.cmd = /bin/bash'
~$ lxc start rhel7
~$ lxc console --show-log rhel7
Console log:
[root@rhel7 /]#

Now that the container has started, you can check it and see that things are not running as well as expected:

~$ lxc exec rhel7 -- bash
[root@rhel7 ~]# ls
[root@rhel7 ~]# mount
mount: failed to read mtab: No such file or directory
[root@rhel7 ~]# cd /
[root@rhel7 /]# ls /proc/
sys
[root@rhel7 /]# exit

Because LXD tries to auto-heal, it created some of the directories when it was starting up. Shutting down and restarting the container fixes the problem, but the original cause is still there - the template does not contain the required files.