[lfs-dev] systemd vs system V

Armin K. krejzi at email.com
Mon Mar 31 12:28:20 PDT 2014


On 03/31/2014 09:20 PM, Pierre Labastie wrote:
> Le 31/03/2014 20:26, Bruce Dubbs a écrit :
>> 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.
>>
>>
> -You have three points in "two of the points of traditional Unix
> philosophy"... Maybe violates ids to strong?
> 

I still believe that UNIX Philosophy is kind of wrong thing to "believe
in" in these modern days. Lots of modern software, especially GUI
software can't really do "one thing". Also, today's hardware is not
really limited as in the era of UNIX so I rather find it ridiculous. But
I can't argue that fact is valid for systemd and many other programs.

> - Pointers to the "extensive documentation"?
> 

http://www.freedesktop.org/wiki/Software/systemd/

Scroll down to:

"Manuals and Documentation for Users and Administrators"

> - Serial processing of boot tasks is traditional in sysv, but not mandatory.
> One could imagine creating some Makefile style processing, (actually, Debian
> does, I think). Then if the equivalent of make is called with the equivalent
> of -jN, some parallelism could be achieved.
> 
> s/extablished/established
> s/diabled/disabled/
> 
> Regards
> Pierre
> 


-- 
Note: My last name is not Krejzi.



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