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

    Enter the following command to display the instance log:

    lxc info <instance_name> --show-log
    Console log

    Enter the following command to display the console log:

    lxc console <instance_name> --show-log
    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. Make sure to include the log files you collected.

Troubleshooting example

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

user@host:~$ lxc console --show-log systemd
Console log: Failed to insert module 'autofs4'Failed to insert module 'unix'Failed to mount sysfs at /sys: Operation not permittedFailed 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 systemd raw.lxc 'lxc.init.cmd = /bin/bash'

Here is what it looks like:

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

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

user@host:~$ lxc exec systemd -- bash
[root@systemd ~]# ls[root@systemd ~]# mountmount: failed to read mtab: No such file or directory[root@systemd ~]# cd /[root@systemd /]# ls /proc/sys[root@systemd /]# 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.