Re: What is systemd
Depending on who you talk to, Systemd is either the spawn of Satan, or it's the salvation of software packagers. More specifically, it's a type of "init system".
During boot up, an init system (there are several different ones used in various LInux distros) gets called to start up everything else and to shut them back down again. Debian currently uses a System V type init, which is based around shell scripts. It goes way back in Unix history, although the implementation details vary. The name comes from System V Unix, which was an attempted unification of AT&T Unix and BSD. Ubuntu uses something called Upstart (Red Hat used to use it as well, until their latest release), and Gentoo has their own called OpenRC.
Systemd is basically a copy of the Apple OS/X init system, which in turn is a copy of the Sun init system. I'm using the term "copy" rather loosely, as init systems have to be closely tailored to the actual OS, and Systemd naturally has lots of Linux specific bits to it. However, the basic way of doing things is similar with regards to when and how things are started up in all three.
Opposition to Systemd tends to revolve around two things, and it should be noted that not everyone who dislikes it, dislikes it for the same reasons.
Some people dislike it simply because it's not System V. These are the "grey beards" that Systemd proponents like to talk about. Other people dislike it because they think it's a good concept which has been badly implemented by idiots who ignore valid criticism. The latter are the people whom the Systemd proponents don't like to admit exist, and they may in fact form the majority of the opposition.
With Debian, there is an additional complication. There are, or rather were, two versions of the Debian OS in development which were not based on Linux. One used a BSD kernel, and the other used a Hurd kernel (a micro-kernel). Systemd kills both of these as Debian projects because the Systemd developers insist on inserting their tentacles into nearly everything, and won't accept patches which will make Systemd cross-platform (e.g, make it compatible with a BSD kernel). Systemd is to be Linux only, Gnome and KDE are to be Systemd only, and anything that Systemd touches will no longer work on BSD (or Hurd) without forking them. This is also going to cause major pain for BSD in general in the future by the way.
The main reason why Debian is changing to Systemd has nothing to do with the relative merits (or demerits) of it versus System V. There is no commercial Debian corporation. Debian developers are generally people who do things with software (e.g. consultants, system administrators, etc.), and cooperate to produce an OS which isn't under the thumb of a particular commercial company. Each Debian developer is responsible for maintaining a specific package or set of packages. They are wildly successful in that Debian is pretty widely used in the server world, and the majority of other Linux distros (e.g. Ubuntu, Mint, as well as many others) are derivatives of it (a derivative being different from a fork in that it keeps going back to the original well for each release).
As such, Debian developers mainly just repackage, test, and fix minor issues with third party software rather than developing anything new themselves. The Systemd developers (basically, Red Hat) have managed to insert hard dependencies on Systemd into other critical Linux components (e.g. the Gnome GUI desktop, also controlled by Red Hat), meaning that not using Systemd would require effort which the Debian developers are not interested in doing, and don't have the resources for anyway. The discussion which Debian developers have had can basically be summed up as "keeping System V sounds nice, but who's going to do the work for it - because I don't have the time". Canonical (Ubuntu) have already said that they'll just go along with whatever Debian decides to do, so Ubuntu will switch from Upstart to Systemd, although that may not happen for several years.
The Devuan developers are going to have to address the resource issues as noted above. Good luck to them though if they succeed and give some people what they want. There are already loads of independent Debian derivatives (which I think Devuan is more like, rather than a true "fork"), which focus on things which aren't addressed by the main release, so one more isn't a problem.
Biting the hand that feeds IT
