I've been working on rewriting Chapter 7 to incorporate systemd. I've
come up with the following text in the introduction and would like
feedback. Thanks,
-- Bruce
7.1.1. System V
System V is the classic boot process that has been used in Unix and
Unix-like systems such as Linux since about 1983. It consists of a small
program, init, that sets up basic programs such as login (via getty) and
runs a script. This script, usually named rc, controls the execution of
a set of additional scripts that perform the tasks required to
initialize the system.
The init program is controlled by the /etc/inittab file and is organized
into run levels that can be run by the user:
0 — halt
1 — Single user mode
2 — Multiuser, without networking
3 — Full multiuser mode
4 — User definable
5 — Full multiuser mode with display manager
6 — reboot
The usual default run level is 3 or 5.
Advantages
+ Established, well understood system.
+ Easy to customize.
Disadvantages
- Slower to boot. A medium speed base LFS system takes 8-12 seconds
where the boot time is measured from the first kernel message to the
login prompt. Network connectivity is typically established about 2
seconds after the login prompt.
- Serial processing of boot tasks. This is related to the previous
point. A delay in any process such as a file system check, will deleay
the entire boot process.
- Does not directly support advanced features like control groups
(cgroups), and per-user fair share scheduling.
- Adding scripts requires manual, static sequencing decisions.
7.1.2. Systemd
Systemd is a group of interconnected programs that handles system and
individual process requests. It provides a dependency system between
various entities called "units". It automatically addresses dependencies
between units and can execute several startup tasks in parallel. It
provides login, inetd, logging, time, networking services, and other tasks.
Advantages
+ Used on many extablished distributions by default.
+ There is extensive documentation.
+ Parallel execution of boot processes. A medium speed base LFS
system takes 6-10 seconds from kernel start to a login prompt. Network
connectivity is typically established about 2 seconds after the login
prompt. More complex startup procedures may show a greater speedup when
compared to System V.
+ Implements advanced features such as control groups to manage
related processes.
+ Maintains backward compatibility with System V programs and
scripts.
Disadvantages
- There is a substantial learning curve.
- Some advanced features such as dbus or cgroups cannot be diabled
if they are not otherwise needed. Systemd knows better than the user.
- Logging is done in a binary format. Extra tools must be used to
process logs or additional processes must be implemented to duplicate
traditional logging programs.
- Systemd violates two of the points of traditional Unix philosophy:
Write programs that do one thing and do it well.
Write programs to work together.
Write programs to handle text streams, because that is a
universal interface.
--
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