HPR1930: A systemd primer

Summary: 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 netw