HPR1930: A systemd primer

1 What is systemd?

A dependency system for unix services.
And, a set of basic unix services to make a unix system usable.
And, a growing list of not quite so basic services

• NTP, networkd, timers (crond/atd)

From a programmers perspective, it's the mainloop phenomenon.

2 Alternatives

Solaris: Service Management Facility
Mac OSX: launchd
Ubuntu: upstart (until recently)

3 Replaces

SYSV
LSB (actually implements LSB deps)

4 Terminology

units

• [auto]mount
• swap
• path (inotify triggers)
• socket
• timer (crond/atd)
• service
• slice (cgroup)
• pseudo
  • device
  • snapshot
  • scope

targets

• replace run levels
• default target at boot
• can isolate to just one target

5 Advantages - Design

Proper, explicit dependencies between system compontents
Starts components in parallel
A proper separation of concerns, lots of situations covered.

• configuration files are regular, simple to understand generally small
• OTOH, there are LOTS of options

Configuration is not runnable shell.

[Unit]
Description=CUPS Scheduler
Documentation=man:cupsd(8)

[Service]
ExecStart=/usr/sbin/cupsd -l
Type=simple

[Install]
Also=cups.socket cups.path
WantedBy=printer.target

Separate system and user daemons.

6 Advantages - Sysadmins

Modify configuration without modifying upstream configuration
Service watching (startup, watchdog, failure modes)
systemd-delta

[EXTENDED]   /lib/systemd/system/rc-local.service → /lib/systemd/system/rc-local.service.d/debian.conf
[EXTENDED]   /lib/systemd/system/systemd-timesyncd.service → /lib/systemd/system/systemd-timesyncd.service.d/disable-with-time-daemon.conf
[EQUIVALENT] /etc/systemd/system/default.target → /lib/systemd/system/default.target

3 overridden configuration files found.

7 Advantages - Programming

Removal of some error and security prone code

• socket activation (e.g. privileged ports)
• user/group changing

8 Advantages - Provisioning

standardized cgroup controls
containers
debootstrap ; systemd-spawn-boot * systemd takes care of all pseudo file systems for you

9 Advantages - Users

quick to boot
can reduce load later on (services start & stop as required)

10 Examples

Color legend:

• black = Requires
• dark blue = Requisite
• dark grey = Wants
• red = Conflicts
• green = After

systemd-analyze blame

systemd-analyze plot

systemd-analyze plot gdm.service

11 Disadvantages

Journald

• It’s really nice in theory, but in practice I’ve found it to be slow and buggy

It’s a little new, so LTS distros necessarily have older versions

• el7 has something like 200 patches

network-online.target is a bit flakey
DBUS

• Unix is a graveyard of IPC, I don't feel DBUS is much better
• KDBUS means it will probably be around for ever.

12 Quandries

Deeply hooked into linux specific details, not portable

• kernel api, cgroups, udev etc.

Some cool features relient on file system e.g. btrfs for snapshot

13 Future

I haven’t had a chance to play with networkd yet, but it sounds like it’s going to be very good.

14 Questions

Migrating

• It depends…
• systemd only supports start/stop/reload
• work with the daemon: oneshot/simple/forking/inetd
• integrate with systemd: notify, watchdog

Userspace

• Every login, a separate systemd -> user is spawned
• Can override with .config/systemd files

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