Post Syndicated from Lennart Poettering original https://0pointer.net/blog/projects/the-biggest-myths.html
Since we first proposed systemd
for inclusion in the distributions it has been frequently discussed in
many forums, mailing lists and conferences. In these discussions one
can often hear certain myths about systemd, that are repeated over and
over again, but certainly don’t gain any truth by constant
repetition. Let’s take the time to debunk a few of them:
-
Myth: systemd is monolithic.
If you build systemd with all configuration options enabled you
will build 69 individual binaries. These binaries all serve different
tasks, and are neatly separated for a number of reasons. For example,
we designed systemd with security in mind, hence most daemons run at
minimal privileges (using kernel capabilities, for example) and are
responsible for very specific tasks only, to minimize their security
surface and impact. Also, systemd parallelizes the boot more than any
prior solution. This parallization happens by running more processes
in parallel. Thus it is essential that systemd is nicely split up into
many binaries and thus processes. In fact, many of these
binaries[1] are separated out so nicely, that they are very
useful outside of systemd, too.TrendingA package involving 69 individual binaries can hardly be called
monolithic. What is different from prior solutions however,
is that we ship more components in a single tarball, and maintain them
upstream in a single repository with a unified release cycle. -
Myth: systemd is about speed.
Yes, systemd is fast (A
pretty complete userspace boot-up in ~900ms, anyone?), but that’s
primarily just a side-effect of doing things right. In fact, we
never really sat down and optimized the last tiny bit of performance
out of systemd. Instead, we actually frequently knowingly picked the
slightly slower code paths in order to keep the code more
readable. This doesn’t mean being fast was irrelevant for us, but
reducing systemd to its speed is certainly quite a misconception,
since that is certainly not anywhere near the top of our list of
goals. -
Myth: systemd’s fast boot-up is irrelevant for
servers.That is just completely not true. Many administrators actually are
keen on reduced downtimes during maintenance windows. In High
Availability setups it’s kinda nice if the failed machine comes back
up really fast. In cloud setups with a large number of VMs or
containers the price of slow boots multiplies with the number of
instances. Spending minutes of CPU and IO on really slow boots of
hundreds of VMs or containers reduces your system’s density
drastically, heck, it even costs you more energy. Slow boots can be
quite financially expensive. Then, fast booting of containers allows
you to implement a logic such as socket
activated containers, allowing you to drastically increase the
density of your cloud system.Of course, in many server setups boot-up is indeed irrelevant, but
systemd is supposed to cover the whole range. And yes, I am aware
that often it is the server firmware that costs the most time at
boot-up, and the OS anyways fast compared to that, but well, systemd
is still supposed to cover the whole range (see above…), and no,
not all servers have such bad firmware, and certainly not VMs and
containers, which are servers of a kind, too.[2] -
Myth: systemd is incompatible with shell scripts.
This is entirely bogus. We just don’t use them for the boot
process, because we believe they aren’t the best tool for that
specific purpose, but that doesn’t mean systemd was incompatible with
them. You can easily run shell scripts as systemd services, heck, you
can run scripts written in any language as systemd services,
systemd doesn’t care the slightest bit what’s inside your
executable. Moreover, we heavily use shell scripts for our own
purposes, for installing, building, testing systemd. And you can stick
your scripts in the early boot process, use them for normal services,
you can run them at latest shutdown, there are practically no
limits. -
Myth: systemd is difficult.
This also is entire non-sense. A systemd platform is actually much
simpler than traditional Linuxes because it unifies
system objects and their dependencies as systemd units. The
configuration file language is very simple, and redundant
configuration files we got rid of. We provide uniform tools for much
of the configuration of the system. The system is much less
conglomerate than traditional Linuxes are. We also have pretty
comprehensive documentation (all linked
from the homepage) about pretty much every detail of systemd, and
this not only covers admin/user-facing interfaces, but also developer
APIs.systemd certainly comes with a learning curve. Everything
does. However, we like to believe that it is actually simpler to
understand systemd than a Shell-based boot for most people. Surprised
we say that? Well, as it turns out, Shell is not a pretty language to
learn, it’s syntax is arcane and complex. systemd unit files are
substantially easier to understand, they do not expose a programming
language, but are simple and declarative by nature. That all said, if
you are experienced in shell, then yes, adopting systemd will take a
bit of learning.To make learning easy we tried hard to provide the maximum
compatibility to previous solutions. But not only that, on many
distributions you’ll find that some of the traditional tools will now
even tell you — while executing what you are asking for — how you
could do it with the newer tools instead, in a possibly nicer way.Anyway, the take-away is probably that systemd is probably as
simple as such a system can be, and that we try hard to make it easy
to learn. But yes, if you know sysvinit then adopting systemd will
require a bit learning, but quite frankly if you mastered sysvinit,
then systemd should be easy for you. -
Myth: systemd is not modular.
Not true at all. At compile time you have a number of
configure switches to select what you want to build, and what
not. And we
document how you can select in even more detail what you need,
going beyond our configure switches.This modularity is not totally unlike the one of the Linux kernel,
where you can select many features individually at compile time. If the
kernel is modular enough for you then systemd should be pretty close,
too. -
Myth: systemd is only for desktops.
That is certainly not true. With systemd we try to cover pretty
much the same range as Linux itself does. While we care for desktop
uses, we also care pretty much the same way for server uses, and
embedded uses as well. You can bet that Red Hat wouldn’t make it a
core piece of RHEL7 if it wasn’t the best option for managing services
on servers.People from numerous companies work on systemd. Car manufactureres
build it into cars, Red Hat uses it for a server operating system, and
GNOME uses many of its interfaces for improving the desktop. You find
it in toys, in space telescopes, and in wind turbines.Most features I most recently worked on are probably relevant
primarily on servers, such as container
support, resource
management or the security
features. We cover desktop systems pretty well already, and there
are number of companies doing systemd development for embedded, some
even offer consulting services in it. -
Myth: systemd was created as result of the NIH syndrome.
This is not true. Before we began working on systemd we were
pushing for Canonical’s Upstart to be widely adopted (and Fedora/RHEL
used it too for a while). However, we eventually came to the
conclusion that its design was inherently flawed at its core (at least
in our eyes: most fundamentally, it leaves dependency management to
the admin/developer, instead of solving this hard problem in code),
and if something’s wrong in the core you better replace it, rather
than fix it. This was hardly the only reason though, other things that
came into play, such as the licensing/contribution agreement mess
around it. NIH wasn’t one of the reasons, though…[3] -
Myth: systemd is a freedesktop.org project.
Well, systemd is certainly hosted at fdo, but freedesktop.org is
little else but a repository for code and documentation. Pretty much
any coder can request a repository there and dump his stuff there (as
long as it’s somewhat relevant for the infrastructure of free
systems). There’s no cabal involved, no “standardization” scheme, no
project vetting, nothing. It’s just a nice, free, reliable place to
have your repository. In that regard it’s a bit like SourceForge,
github, kernel.org, just not commercial and without over-the-top
requirements, and hence a good place to keep our stuff.So yes, we host our stuff at fdo, but the implied assumption of
this myth in that there was a group of people who meet and then agree
on how the future free systems look like, is entirely bogus. -
Myth: systemd is not UNIX.
There’s certainly some truth in that. systemd’s sources do not
contain a single line of code originating from original UNIX. However,
we derive inspiration from UNIX, and thus there’s a ton of UNIX in
systemd. For example, the UNIX idea of “everything is a file” finds
reflection in that in systemd all services are exposed at runtime in a
kernel file system, the cgroupfs. Then, one of the original
features of UNIX was multi-seat support, based on built-in terminal
support. Text terminals are hardly the state of the art how you
interface with your computer these days however. With systemd we
brought native multi-seat
support back, but this time with full support for today’s hardware,
covering graphics, mice, audio, webcams and more, and all that fully
automatic, hotplug-capable and without configuration. In fact the
design of systemd as a suite of integrated tools that each have their
individual purposes but when used together are more than just the sum
of the parts, that’s pretty much at the core of UNIX philosophy. Then,
the way our project is handled (i.e. maintaining much of the core OS
in a single git repository) is much closer to the BSD model (which is
a true UNIX, unlike Linux) of doing things (where most of the core OS
is kept in a single CVS/SVN repository) than things on Linux ever
were.Ultimately, UNIX is something different for everybody. For us
systemd maintainers it is something we derive inspiration from. For
others it is a religion, and much like the other world religions there
are different readings and understandings of it. Some define UNIX
based on specific pieces of code heritage, others see it just as a set
of ideas, others as a set of commands or APIs, and even others as a
definition of behaviours. Of course, it is impossible to ever make all
these people happy.Ultimately the question whether something is UNIX or not matters
very little. Being technically excellent is hardly exclusive to
UNIX. For us, UNIX is a major influence (heck, the biggest one), but
we also have other influences. Hence in some areas systemd will be
very UNIXy, and in others a little bit less. -
Myth: systemd is complex.
There’s certainly some truth in that. Modern computers are complex
beasts, and the OS running on it will hence have to be complex
too. However, systemd is certainly not more complex than prior
implementations of the same components. Much rather, it’s simpler, and
has less redundancy (see above). Moreover, building a simple OS based
on systemd will involve much fewer packages than a traditional Linux
did. Fewer packages makes it easier to build your system, gets rid of
interdependencies and of much of the different behaviour of every
component involved. -
Myth: systemd is bloated.
Well, bloated certainly has many different definitions. But in
most definitions systemd is probably the opposite of bloat. Since
systemd components share a common code base, they tend to share much
more code for common code paths. Here’s an example: in a traditional
Linux setup, sysvinit, start-stop-daemon, inetd, cron, dbus, all
implemented a scheme to execute processes with various configuration
options in a certain, hopefully clean environment. On systemd the code
paths for all of this, for the configuration parsing, as well as the
actual execution is shared. This means less code, less place for
mistakes, less memory and cache pressure, and is thus a very good
thing. And as a side-effect you actually get a ton more functionality
for it…As mentioned above, systemd is also pretty modular. You can choose
at build time which components you need, and which you don’t
need. People can hence specifically choose the level of “bloat” they
want.When you build systemd, it only requires three dependencies: glibc,
libcap and dbus. That’s it. It can make use of more dependencies, but
these are entirely optional.So, yeah, whichever way you look at it, it’s really not
bloated. -
Myth: systemd being Linux-only is not nice to the BSDs.
Completely wrong. The BSD folks are pretty much uninterested in
systemd. If systemd was portable, this would change nothing, they
still wouldn’t adopt it. And the same is true for the other Unixes in
the world. Solaris has SMF, BSD has their own “rc” system, and they
always maintained it separately from Linux. The init system is very
close to the core of the entire OS. And these other operating systems
hence define themselves among other things by their core
userspace. The assumption that they’d adopt our core userspace if we
just made it portable, is completely without any foundation. -
Myth: systemd being Linux-only makes it impossible for Debian to adopt it as default.
Debian supports non-Linux kernels in their distribution. systemd
won’t run on those. Is that a problem though, and should that hinder
them to adopt system as default? Not really. The folks who ported
Debian to these other kernels were willing to invest time in a massive
porting effort, they set up test and build systems, and patched and
built numerous packages for their goal. The maintainance of both a
systemd unit file and a classic init script for the packaged services
is a negligable amount of work compared to that, especially since
those scripts more often than not exist already. -
Myth: systemd could be ported to other kernels if its maintainers just wanted to.
That is simply not true. Porting systemd to other kernel is not
feasible. We just use too many Linux-specific interfaces. For a few
one might find replacements on other kernels, some features one might
want to turn off, but for most this is nor really possible. Here’s a
small, very incomprehensive list: cgroups, fanotify, umount2(),
/proc/self/mountinfo (including notification), /dev/swaps (same),
udev, netlink, the structure of /sys, /proc/$PID/comm,
/proc/$PID/cmdline, /proc/$PID/loginuid, /proc/$PID/stat,
/proc/$PID/session, /proc/$PID/exe, /proc/$PID/fd, tmpfs, devtmpfs,
capabilities, namespaces of all kinds, various prctl()s, numerous
ioctls, the mount() system call and its semantics, selinux, audit,
inotify, statfs, O_DIRECTORY, O_NOATIME, /proc/$PID/root, waitid(),
SCM_CREDENTIALS, SCM_RIGHTS, mkostemp(), /dev/input, ...And no, if you look at this list and pick out the few where you can
think of obvious counterparts on other kernels, then think again, and
look at the others you didn’t pick, and the complexity of replacing
them. -
Myth: systemd is not portable for no reason.
Non-sense! We use the Linux-specific functionality because we need
it to implement what we want. Linux has so many features that
UNIX/POSIX didn’t have, and we want to empower the user with
them. These features are incredibly useful, but only if they are
actually exposed in a friendly way to the user, and that’s what we do
with systemd. -
Myth: systemd uses binary configuration files.
No idea who came up with this crazy myth, but it’s absolutely not
true. systemd is configured pretty much exclusively via simple text
files. A few settings you can also alter with the kernel command line
and via environment variables. There’s nothing binary in its
configuration (not even XML). Just plain, simple, easy-to-read text
files. -
Myth: systemd is a feature creep.
Well, systemd certainly covers more ground that it used to. It’s
not just an init system anymore, but the basic userspace building
block to build an OS from, but we carefully make sure to keep most of
the features optional. You can turn a lot off at compile time, and
even more at runtime. Thus you can choose freely how much feature
creeping you want. -
Myth: systemd forces you to do something.
systemd is not the mafia. It’s Free Software, you can do with it
whatever you want, and that includes not using it. That’s pretty much
the opposite of “forcing”. -
Myth: systemd makes it impossible to run syslog.
Not true, we carefully made sure when we introduced
the journal that all data is also passed on to any syslog daemon
running. In fact, if something changed, then only that syslog gets
more complete data now than it got before, since we now cover early
boot stuff as well as STDOUT/STDERR of any system service. -
Myth: systemd is incompatible.
We try very hard to provide the best possible compatibility with
sysvinit. In fact, the vast majority of init scripts should work just
fine on systemd, unmodified. However, there actually are indeed a few
incompatibilities, but we try to document
these and explain what to do about them. Ultimately every system
that is not actually sysvinit itself will have a certain amount of
incompatibilities with it since it will not share the exect same code
paths.It is our goal to ensure that differences between the various
distributions are kept at a minimum. That means unit files usually
work just fine on a different distribution than you wrote it on, which
is a big improvement over classic init scripts which are very hard to
write in a way that they run on multiple Linux distributions, due to
numerous incompatibilities between them. -
Myth: systemd is not scriptable, because of its D-Bus use.
Not true. Pretty much every single D-Bus interface systemd provides
is also available in a command line tool, for example in systemctl,
loginctl,
timedatectl,
hostnamectl,
localectl
and suchlike. You can easily call these tools from shell scripts, they
open up pretty much the entire API from the command line with
easy-to-use commands.That said, D-Bus actually has bindings for almost any scripting
language this world knows. Even from the shell you can invoke
arbitrary D-Bus methods with dbus-send
or gdbus. If
anything, this improves scriptability due to the good support of D-Bus
in the various scripting languages. -
Myth: systemd requires you to use some arcane configuration
tools instead of allowing you to edit your configuration files
directly.Not true at all. We offer some configuration tools, and using them
gets you a bit of additional functionality (for example, command line
completion for all settings!), but there’s no need at all to use
them. You can always edit the files in question directly if you wish,
and that’s fully supported. Of course sometimes you need to explicitly
reload configuration of some daemon after editing the configuration,
but that’s pretty much true for most UNIX services. -
Myth: systemd is unstable and buggy.
Certainly not according to our data. We have been monitoring the
Fedora bug tracker (and some others) closely for a long long time. The
number of bugs is very low for such a central component of the OS,
especially if you discount the numerous RFE bugs we track for the
project. We are pretty good in keeping systemd out of the list of
blocker bugs of the distribution. We have a relatively fast
development cycle with mostly incremental changes to keep quality and
stability high. -
Myth: systemd is not debuggable.
False. Some people try to imply that the shell was a good
debugger. Well, it isn’t really. In systemd we provide you with actual
debugging features instead. For example: interactive debugging,
verbose tracing, the ability to mask any component during boot, and
more. Also, we provide documentation
for it.It’s certainly well debuggable, we needed that for our own
development work, after all. But we’ll grant you one thing: it uses
different debugging tools, we believe more appropriate ones for the
purpose, though. -
Myth: systemd makes changes for the changes’ sake.
Very much untrue. We pretty much exclusively have technical
reasons for the changes we make, and we explain them in the various
pieces of documentation, wiki pages, blog articles, mailing list
announcements. We try hard to avoid making incompatible changes, and
if we do we try to document the why and how in detail. And if you
wonder about something, just ask us! -
Myth: systemd is a Red-Hat-only project, is private property
of some smart-ass developers, who use it to push their views to the
world.Not true. Currently, there are 16 hackers with commit powers to the
systemd git tree. Of these 16 only six are employed by Red Hat. The 10
others are folks from ArchLinux, from Debian, from Intel, even from
Canonical, Mandriva, Pantheon and a number of community folks with
full commit rights. And they frequently commit big stuff, major
changes. Then, there are 374 individuals with patches in our tree, and
they too came from a number of different companies and backgrounds,
and many of those have way more than one patch in the tree. The
discussions about where we want to take systemd are done in the open,
on our IRC channel (#systemd on freenode, you are always
weclome), on our mailing
list, and on public hackfests (such
as our next one in Brno, you are invited). We regularly attend
various conferences, to collect feedback, to explain what we are doing
and why, like few others do. We maintain blogs, engage in social
networks (we actually
have some pretty interesting content on Google+, and our Google+
Community is pretty alive, too.), and try really hard to explain
the why and the how how we do things, and to listen to feedback and
figure out where the current issues are (for example, from that
feedback we compiled this lists of often heard myths about
systemd…).What most systemd contributors probably share is a rough idea how a
good OS should look like, and the desire to make it happen. However,
by the very nature of the project being Open Source, and rooted in the
community systemd is just what people want it to be, and if it’s not
what they want then they can drive the direction with patches and
code, and if that’s not feasible, then there are numerous other
options to use, too, systemd is never exclusive.One goal of systemd is to unify the dispersed Linux landscape a
bit. We try to get rid of many of the more pointless differences of
the various distributions in various areas of the core OS. As part of
that we sometimes adopt schemes that were previously used by only one
of the distributions and push it to a level where it’s the default of
systemd, trying to gently push everybody towards the same set of basic
configuration. This is never exclusive though, distributions can
continue to deviate from that if they wish, however, if they end-up
using the well-supported default their work becomes much easier and
they might gain a feature or two. Now, as it turns out, more
frequently than not we actually adopted schemes that where Debianisms,
rather than Fedoraisms/Redhatisms as best supported scheme by
systemd. For example, systems running systemd now generally store
their hostname in /etc/hostname, something that used to be
specific to Debian and now is used across distributions.One thing we’ll grant you though, we sometimes can be
smart-asses. We try to be prepared whenever we open our mouth, in
order to be able to back-up with facts what we claim. That might make
us appear as smart-asses.But in general, yes, some of the more influental contributors of
systemd work for Red Hat, but they are in the minority, and systemd is
a healthy, open community with different interests, different
backgrounds, just unified by a few rough ideas where the trip should
go, a community where code and its design counts, and certainly not
company affiliation. -
Myth: systemd doesn’t support /usr split from the root directory.
Non-sense. Since its beginnings systemd supports the
--with-rootprefix= option to its configure script
which allows you to tell systemd to neatly split up the stuff needed
for early boot and the stuff needed for later on. All this logic is
fully present and we keep it up-to-date right there in systemd’s build
system.Of course, we still don’t think that actually
booting with /usr unavailable is a good idea, but we
support this just fine in our build system. This won’t fix the
inherent problems of the scheme that you’ll encounter all across the
board, but you can’t blame that on systemd, because in systemd we
support this just fine. -
Myth: systemd doesn’t allow your to replace its components.
Not true, you can turn off and replace pretty much any part of
systemd, with very few exceptions. And those exceptions (such as
journald) generally allow you to run an alternative side by side to
it, while cooperating nicely with it. -
Myth: systemd’s use of D-Bus instead of sockets makes it intransparent.
This claim is already contradictory in itself: D-Bus uses sockets
as transport, too. Hence whenever D-Bus is used to send something
around, a socket is used for that too. D-Bus is mostly a standardized
serialization of messages to send over these sockets. If anything this
makes it more transparent, since this serialization is well
documented, understood and there are numerous tracing tools and
language bindings for it. This is very much unlike the usual
homegrown protocols the various classic UNIX daemons use to
communicate locally.
Hmm, did I write I just wanted to debunk a “few” myths? Maybe these
were more than just a few… Anyway, I hope I managed to clear up a
couple of misconceptions. Thanks for your time.
Footnotes
[1] For example, systemd-detect-virt,
systemd-tmpfiles,
systemd-udevd are.
[2] Also, we are trying to do our little part on maybe
making this better. By exposing boot-time performance of the firmware
more prominently in systemd’s boot output we hope to shame the
firmware writers to clean up their stuff.
[3] And anyways, guess which project includes a library “libnih” — Upstart or systemd?[4]
[4] Hint: it’s not systemd!