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Revision as of 15:40, 11 May 2012 by Michich (talk | contribs) (diagnosing boot problems)


Foreword

If you are experiencing a problem with system boot up due to Systemd, please see the common bugs document before filing a bug. Some easy configuration tweaks that fix a wide range of issues may be listed there. If the problem you are seeing is not listed there or none of the workarounds seem to help, please consider filing a bug to help us make Fedora run better on your hardware.

Be prepared to include some information (logs) about your system as well. These should be complete (no snippets please), not in an archive, uncompressed, with MIME type set as text/plain.

Diagnosing boot problems

If your machine gets stuck during boot, first check if the hang happens before or after control passes to systemd.

Try to boot without rhgb and quiet on the kernel command line. If you see some messages like these:

  • Welcome to Fedora VERSION (codename)!"
  • Starting name...
  • Started name. [ OK ]

then systemd is running.

Problems booting this far?
If you experience bootup difficulties before systemd starts running, see Debugging Dracut to diagnose that further.
Problems running udev?
If you're system hangs while starting or running udev, see Debugging Udev to diagnose that further.

Debugging always gets easier when you can get a shell. If you do not get a login prompt, try switching to a different virtual terminal using CTRL+ALT+F<number>. Problems with X server startup may manifest themselves as a missing login on tty1, but other VTs working.

If the boot stops without presenting you with a login on any virtual console, give it up to 5 minutes before declaring it definitely stuck. There is a chance that a service that has trouble starting will be killed after this timeout and the boot will continue normally. Another possibility is that a device for an important mountpoint will fail to appear and you will be presented with emergency mode.

If you get no shell

If you get neither a normal login nor the emergency mode shell, you will need to do additional steps to get debugging information out of the machine.

  • Try CTRL+ALT+DEL to reboot.
    • If it does not reboot, mention it in your bugreport. Meanwhile force the reboot with SysRq or hard reset.
  • When booting the next time, you will have to add some kernel command line arguments depending on what debugging strategy you choose from the following options.

strategy: debug logging to a serial console

If you have a hardware serial console available or if you are debugging in a virtual machine (e.g. using virt-manager you can switch your view to a serial console in the menu View -> Text Consoles), you can ask systemd to log lots of useful debugging information to it by booting with: 

systemd.log_level=debug systemd.log_target=console console=ttyS0,38400

strategy: booting into rescue or emergency targets

To boot directly into rescue target add systemd.unit=rescue.target or 1 to the kernel command line. This target is useful if the problem occurs somewhere after the basic system is brought up, during the starting of "normal" services. If this is the case, you should be able to disable the bad service from here. If the rescue target will not boot either, the more minimal emergency target might.

To boot directly into emergency shell add systemd.unit=emergency.target or emergency to the kernel command line. Note that in the emergency shell you will have to remount the root filesystem read-write by yourself before editing any files: 

mount -o remount,rw /

Common issues that can be resolved in the emergency shell are bad lines in /etc/fstab. After fixing /etc/fstab, run systemctl daemon-reload to let systemd refresh its view of it.

If not even the emergency target works, you can boot directly into a shell with init=/bin/sh. This may be necessary in case systemd itself or some libraries it depends on are damaged by filesystem corruption. You may need to reinstall working versions of the affected packages.

If /bin/sh does not work, you must boot from another medium.

strategy: early debug shell

You can enable shell access to be available very early in the startup process to fall back on and diagnose systemd related boot up issues with various systemctl commands.

When you can get a shell

When you have systemd running to the extent that it can provide you with a shell, please use it to extract useful information for debugging. Boot with these parameters on the kernel command line: 

systemd.log_level=debug systemd.log_target=kmsg log_buf_len=1M

in order to increase the verbosity of systemd, to let systemd write its logs to the kernel log buffer, and to increase the size of the kernel log buffer. After reaching the shell, save the log: 

dmesg > dmesg.txt

To check for possibly stuck jobs use: 

systemctl list-jobs

A useful command to run is this, which test-runs systemd and calculates the initial transaction: 

/lib/systemd/systemd --test --system --log-level=debug

Information to include in your report

All bug reports

In all cases, the following should be mentioned and attached to your bug report:

  • The exact kernel command-line used if not default. Typically from the bootloader configuration file (e.g. /etc/grub.conf) or from /proc/cmdline
  • A copy of the file /var/log/messages
  • The output of the dmesg command: dmesg > dmesg.txt
  • The output of a systemd dump: systemctl dump > systemd-dump.txt
  • The output of /bin/systemd --test --system --log-level=debug > systemd-test.txt 2>&1

Configure a serial console

Successfully debugging boot up will require some form of console logging during the system boot. This section documents configuring a serial console connection to record boot messages.

First, enable serial console output for both the kernel and the bootloader. Open the file /etc/grub.conf for editing. Below the line timeout=5, add the following: 

serial --unit=0 --speed=38400
terminal --timeout=5 serial console

Also in /etc/grub.conf, add the following boot arguments to the kernel line: 

console=tty0 console=ttyS0,38400

When finished, the /etc/grub.conf file should look similar to the example below. 

default=0
timeout=5
serial --unit=0 --speed=38400
terminal --timeout=5 serial console
title Fedora (2.6.35.4-18.fc14.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.35.4-18.fc14.x86_64 ro root=/dev/mapper/VolGroup-lv_root rd_LVM_LV=VolGroup/lv_root rd_LVM_LV=VolGroup/lv_swap rd_NO_LUKS rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=is-latin1 systemd.log_level=debug systemd.log_target=kmsg console=tty0 console=ttyS0,38400
        initrd /initramfs-2.6.35.4-18.fc14.x86_64.img

More detailed information on how to configure the kernel for console output can be found at [1].

Redirecting non-interactive output
You can redirect all non-interactive output to /dev/kmsg and the kernel will put it out on the console when it reaches the kernel buffer by doing 
exec >/dev/kmsg 2>&1 </dev/console

Boot into rescue mode or to a emergency shell

To boot directly into rescue mode add systemd.unit=rescue.target or 1 to the kernel command line.

To boot directly into emergency shell add systemd.unit=emergency.target or emergency to the kernel command line.

Enable shell access early in systemd

You can enable shell access early in systemd startup process to fall back on and diagnose systemd related boot up issues with various systemctl commands should systemd startup otherwise fail.

Various useful systemd related commands

  • Run systemctl list-jobs

To identify slow boot and look for the jobs that are "running" those jobs are the ones where boot waits for completion on and the ones that listed as "waiting" will be executed only after those which are "running" are completed.


  • Run systemctl list-units -t service --all

To list all available services and their current status


  • Run systemctl list-units -t service

To show all active services


  • Run systemctl status sshd.service

To examine the current runtime status of a service. (In the above example the ssh service)


  • Run systemctl list-units -t target --all

To show all available targets.


  • Run systemctl list-units -t target

To show all active targets.


  • Run systemctl show -p "Wants" multi-user.target

To see which services a target pulls in. ( In the above example the multi-user.target )


  • Run systemd --test --system --unit=multi-user.target

To examine what gets started when when booted into a specific target. ( In the above example the multi-user.target )

Systemd boot parameters

The following boot parameters are also available to further assist with debugging boot issues.

systemd.unit=
Overrides the unit to activate on boot. This may be used to temporarily boot into a different boot unit, for example rescue.target or emergency.target. ( Defaults to default.target. )
systemd.dump_core=
Takes a boolean argument. If true systemd dumps core when it crashes. Otherwise no core dump is created. ( Defaults to true )
systemd.crash_shell=
Takes a boolean argument. If true systemd spawns a shell when it crashes. Otherwise no core dump is created. Defaults to false, for security reasons, as the shell is not protected by any password authentication.
systemd.crash_chvt=
Takes an integer argument. If positive systemd activates the specified virtual terminal when it crashes. ( Defaults to -1 )
systemd.confirm_spawn=
Takes a boolean argument. If true asks for confirmation when spawning processes. ( Defaults to false )
systemd.show_status=
Takes a boolean argument. If true shows terse service status updates on the console during bootup. ( Defaults to true )
systemd.sysv_console=
Takes a boolean argument. If true output of SysV init scripts will be directed to the console. ( Defaults to true, unless quiet is passed as kernel command line option in which case it defaults to false. )
systemd.log_target=
Set log target. Argument must be one of console, syslog, kmsg, syslog-or-kmsg, null.
systemd.log_level=
Set log level. As argument this accepts a numerical log level or the well-known syslog symbolic names (lowercase): emerg, alert, crit, err, warning, notice, info, debug.
systemd.log_color=
Highlight important log messages. Argument is a boolean value. If the argument is omitted it defaults to true.
systemd.log_location=
Include code location in log messages. This is mostly relevant for debugging purposes. Argument is a boolean value. If the argument is omitted it defaults to true.
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