Re: [gentoo-user] Re: Anyone switched to eudev yet? -> what was wron with SysVInit?

[Canek Peláez Valdés]

On Tue, Dec 25, 2012 at 1:38 AM, G.Wolfe Woodbury <redwolfe@×××××.com> wrote:
[ snip ]
> From what has been happening with the systemd stuff, I do not see what
> advantages it really offers over the SysV scheme and its successors like
> OpenRC.  Someone enlighten me please?

I wrote the following some months ago; I think nothing much has
changed since then (I added a couple of comments):

Take this with a grain (or a kilo) of salt, since I'm obviously
biased, but IMHO this are systemd advantages over OpenRC:

* Really fast boot. OpenRC takes at least double the time that systemd
does when booting, easily verifiable. In my laptop systemd is twice as
fast as OpenRC; in my desktop is three times faster. (With a solid
state hard drive, my laptop now boots even faster).

* Really parallel service startup: OpenRC has never been reliable on
parallel service startup; its documentation says it explicitly. Some
will tell you that for them "it works", but just like the guys who
have a separate /usr and refuse to use an initramfs, they just haven't
been bitten by the inherent problems of it (just ask kernel developer
Greg Kroah-Hartman). The Gentoo devs recognize that OpenRC is just
broken with parallel service startup.

* Really simple service unit files: The service unit files are really
small, really simple, really easy to understand/modify. Compare the 9
lines of sshd.service:

$ cat /etc/systemd/system/sshd.service
[Unit]
Description=SSH Secure Shell Service
After=syslog.target

[Service]
ExecStart=/usr/sbin/sshd -D

[Install]
WantedBy=multi-user.target

with the 84 of /etc/init.d/sshd (80 without comments).

* Really good documentation: systemd has one of the best
documentations I have ever seen in *any* project. Everything (except
really new, experimental features) is documented, with manual pages
explaining everything. And besides, there are blog posts by Lennart
explaining in a more informal way how to do neat tricks with systemd.

* Really good in-site customization: The service unit files are
trivially overrided with custom ones for specific installations,
without needing to touch the ones installed by systemd or a program.
With OpenRC, if I modify a /etc/init.d file, chances are I need to
check out my next installation so I can see how the new file differs
from the old one, and adapt the changes to my customized version.

* All the goodies from Control Groups: You can use kernel cgroups to
monitor/control several properties of your daemons, out of the box,
almost no admin effort involved.

* It tries to unify Linux behaviour among distros (some can argue that
this is a bad thing): Using systemd, the same
configurations/techniques work the same in every distribution. No more
need to learn /etc/conf.d, /etc/sysconfig, /etc/default hacks by
different distros.

* Finally, and what I think is the most fundamental difference between
systemd and almost any other init system: The service unit files in
systemd are *declarative*; you tell the daemon *what* to do, not *how*
to do it. If the service files are shell scripts (like in
OpenRC/SysV), everything can spiral out of control really easily. And
it usually does (again, look at sshd; and that one is actully nicely
written, there are all kind of monsters out there abusing the power
that shell gives you).

These are the ones off the top of my head; but what I like the most
about systemd is that it just works, and that it makes a lot of sense
(at least to me).

Most of systemd features can be implemented in OpenRC, although the
speed difference will never be eliminated if OpenRC keeps using shell
files; however, Luca Barbato said that using reentrant busybox the
speed difference is greatly reduced (I haven't confirmed this, since I
haven't even installed OpenRC in months).

Now, this set of (IMO) advantages of systemd over OpenRC pile up over
the advantages of OpenRC over SysV: the most important one (I believe)
is that OpenRC has dependencies, so a service starts only when another
has already started. AFAIK, SysV has lacked this since always.

I don't think I have ever heard anyone saying that we should keep
using SysV; like a lot of Unix legacies, it should just die. OpenRC is
much better, but it still uses a Turing-complete language (and a
really slow one) to simply tell services when to start and when to
stop, and it doesn't reliably keep track of what services are really
still running (anyone who has ever used the "zap" command in OpenRC
knows this).

systemd of course has dependencies, a reliable tracking of service
status (thanks in part to the use of cgroups), and its service files
can't enter in an infinite loop.

Hope it helps.

Regards.
-- 
Canek Peláez Valdés
Posgrado en Ciencia e Ingeniería de la Computación
Universidad Nacional Autónoma de México