A Systemd Primer

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

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

Date: 5/11/2015

Author: Clinton Roy

Created: 2015-11-07 Sat 15:23

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