1. System startup process systemd
The ABC of Linux
System startup process
systemd

2. The ABC of Linux - by Peter Larsen
Agenda
Overview
Sysvinit recap
Systemd goals
Commands and cool stuff
August 2012
The ABC of Linux - by Peter Larsen

3. The ABC of Linux - by Peter Larsen
Overview
System startup happens after kernel has loaded
Kernel will run /init or /sbin/init if found
Every system must have a PID 1 – the parent of all processes
Init is responsible for getting the system running
August 2012
The ABC of Linux - by Peter Larsen

4. The ABC of Linux - by Peter Larsen
SysVInit recap
/etc/inittab - runlevels
/etc/rc.d/
Init, rc0.d, rc1.d, rc2.d, rc3.d, rc4.d, rc5.d, rc6.d
/etc/sysconfig
# service command
# chkconfig command
August 2012
The ABC of Linux - by Peter Larsen

5. So what's wrong with SysVInit?
Well nothing except:
Serial startup
Script based
Sequence/dependency hell
It's old – 25 years!
While supposedly a standard, different distributions implements it differently
August 2012
The ABC of Linux - by Peter Larsen

6. But I know trust SysVInit!
Yes, systemd is new
Learning new stuff is hard
Backwards compatibility with old commands
It's worth it – going from 60 seconds to sub 1 second startup times!
August 2012
The ABC of Linux - by Peter Larsen

7. The ABC of Linux - by Peter Larsen
Systemd goals
Parallel startup
No Scripts!
Better control
More options
Re-designed from ground up
August 2012
The ABC of Linux - by Peter Larsen

8. The ABC of Linux - by Peter Larsen
Systemd features #1
aggressive parallelization capabilities using socket
D-Bus activation for starting services
on-demand starting of daemons
keeps track of processes using Linux cgroups
supports snapshotting and restoring of the system state
August 2012
The ABC of Linux - by Peter Larsen

9. The ABC of Linux - by Peter Larsen
Systemd features #2
maintains mount and automount points
implements an elaborate transactional dependency-based service control logic
Full control of spawn processes
The native configuration files use a syntax that closely follows the well-known .desktop files
August 2012
The ABC of Linux - by Peter Larsen

10. The ABC of Linux - by Peter Larsen
Systemd features #3
Compatibility with SysV init scripts
Support a simple templating/instance mechanism
August 2012
The ABC of Linux - by Peter Larsen

11. The ABC of Linux - by Peter Larsen
Systemd units
Service (can be started, stopped, restarted, reloaded)
Socket: this unit encapsulates a socket in the file-system or on the Internet.
Device: this unit encapsulates a device in the Linux device tree.
Mount: this unit encapsulates a mount point in the file system hierarchy.
service: these are the most obvious kind of unit: daemons that can be started, stopped, restarted, reloaded.
socket: this unit encapsulates a socket in the file-system or on the Internet. systemd currently support AF_INET, AF_INET6, AF_UNIX sockets of the types stream, datagram, and sequential packet. It can also support classic FIFOs as transport. Each socket unit has a matching service unit, that is started if the first connection comes in on the socket or FIFO (e.g. nscd.socket starts nscd.service on an incoming connection).
device: this unit encapsulates a device in the Linux device tree. If a device is marked for this via udev rules, it will be exposed as a device unit in systemd. Properties set with udev can be used as configuration source to set dependencies for device units.
mount: this unit encapsulates a mount point in the file system hierarchy.
August 2012
The ABC of Linux - by Peter Larsen

12. Systemd units - continued
Automount: this unit type encapsulates an automount point in the file system hierarchy.
Target: this unit type is used for logical grouping of units.
Snapshot: similar to target
automount: this unit type encapsulates an automount point in the file system hierarchy. Each automount unit has a matching mount unit, which is started (i.e. mounted) as soon as the automount directory is accessed.
target: this unit type is used for logical grouping of units: instead of actually doing anything by itself it simply references other units, which thereby can be controlled together. (e.g. multi-user.target, which is a target that basically plays the role of run-level 5 on classic SysV system; or bluetooth.target which is requested as soon as a bluetooth dongle becomes available and which simply pulls in bluetooth related services that otherwise would not need to be started: bluetoothd and obexd and suchlike).
snapshot: similar to target units snapshots do not actually do anything themselves and their only purpose is to reference other units.
August 2012
The ABC of Linux - by Peter Larsen

13. Commands and Cool Stuff
# systemctl
# systemadm #
August 2012
The ABC of Linux - by Peter Larsen

14. The ABC of Linux - by Peter Larsen
Switching runlevel
# systemctl isolate multi-user.target
# systemctl isolate graphical.target
(or use runlevel3.target and runlevel5.target respectfully)
ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
August 2012
The ABC of Linux - by Peter Larsen

15. The ABC of Linux - by Peter Larsen
Boot parameters
systemd.unit – overwriting default boot mode
systemd.unit=rescue.target
systemd.unit=emergency.target
systemd.unit=multi-user.target
systemd.unit=graphical.target
August 2012
The ABC of Linux - by Peter Larsen

16. The ABC of Linux - by Peter Larsen
Thank you – Questions?
Documentation: docs.html
August 2012
The ABC of Linux - by Peter Larsen