170 lines
4.6 KiB
Text
170 lines
4.6 KiB
Text
Currently available init systems and their problems
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===================================================
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Nico Schottelius <nico-cinit__@__schottelius.org>
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0.1, for cinit 0.3, Initial version from 2006-12-11
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:Author Initials: NS
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This document describes some other init systems, especially
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the "traditional" ones and describes their problems.
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Introduction
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-------------
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some history, sysv, bsd,
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minit, runit, initng, cinit, upstart
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Implementations
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---------------
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List of current implementations as of 2006-12-12.
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Linux Sys-V-Init
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~~~~~~~~~~~~~~~~
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Debian Sid
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/etc/inittab
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"First script" /etc/init.d/rcS (specified
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/etc/init.d/rcS: Bourne shell script text executable
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Runlevels: 0-6, different states
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--------------------------------------------------------------------------------
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[9:51] hydrogenium:cinit-0.3pre6% /sbin/runlevel
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N 2
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--------------------------------------------------------------------------------
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Depending on runlevel, the init calls /etc/init.d/rc $runlevel.
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This is manually configured in /etc/inittab
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/etc/init.d/rc: Bourne shell script text executable
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/etc/init.d/rc
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Calls symlinks in /etc/rc?.d. Each runlevel a directory:
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--------------------------------------------------------------------------------
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[9:51] hydrogenium:cinit-0.3pre6% ls -ld /etc/rc*.d
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drwxr-xr-x 2 root root 4096 2006-12-01 16:56 /etc/rc0.d
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drwxr-xr-x 2 root root 4096 2006-12-01 16:56 /etc/rc1.d
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drwxr-xr-x 3 root root 4096 2006-12-01 16:56 /etc/rc2.d
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drwxr-xr-x 2 root root 4096 2006-12-01 16:56 /etc/rc3.d
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drwxr-xr-x 2 root root 4096 2006-12-01 16:56 /etc/rc4.d
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drwxr-xr-x 2 root root 4096 2006-12-01 16:56 /etc/rc5.d
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drwxr-xr-x 2 root root 4096 2006-12-01 16:56 /etc/rc6.d
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drwxr-xr-x 2 root root 4096 2006-11-28 23:17 /etc/rcS.d
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--------------------------------------------------------------------------------
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rlmanager: sys-v-init hack
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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rlmanager: one script to be executed, sources
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simply configuration
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Gentoo Linux
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~~~~~~~~~~~~
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no content yet
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BSD init
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~~~~~~~~
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Analyzed on FreeBSD.
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/etc/rc
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/etc/rc.d/ system services
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/usr/local/etc/rc.d/ user installed services
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Scripts may use values from /etc/rc.conf to check whether
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they should be started or not.
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runit
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~~~~~
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Three stages: Script1, dependencies, Script3
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Initng
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~~~~~~
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no content yet
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minit
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~~~~~
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Focus on being small. Needs libowfat.
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Log support through pipe.
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Problems overview
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-----------------
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Scripts
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~~~~~~~
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Scripts are slow. Startup time. Execution time. Unecessary overhead.
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Sequential startup
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~~~~~~~~~~~~~~~~~~
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In the 'old' init systems there is no support of parallel execution.
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So if one services hangs at startup, the whole system has to wait
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(for instance a dhcp request). In the worst case this may result in
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a non-booting system. Systems with dependencies can start the gettys
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for login before, in parallel to or even after blocking services are
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started. The fact, that they *are* started means you can interact with
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the system, you can end looping or blocking services.
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So sequential startup may not only be the reason for a pretty
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slow system, but also for an unmaintainable one.
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Unecessary services
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~~~~~~~~~~~~~~~~~~~
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Many implementations try to do whatever-is-possible on boot, trying to
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cover every possible and impossible situation.
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Portmap without any program that needs it.
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Starting all installed services per default (Debian) vs. let the
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user choose what to start (Gentoo).
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Nicht entfernte Altlasten ab und zu.
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No dependencies
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~~~~~~~~~~~~~~~
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Systems without dependencies may result in a chaos state after booting up.
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Only soft dependencies
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~~~~~~~~~~~~~~~~~~~~~~~
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The dependencies are used, but because of soft dependencies, services
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may be started although their dependency is missing.
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Parallel init system debugging
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In contrast to the old init systems, parallel executing init systems
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are much harder to debug. If the starting init systems does not have
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proper logging / reporting to the user (like cinit-0.2 has), the user
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may never find out, what happened and why.
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Parallel / dependency checking init system with variable boot order
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The boot order may be different each time you boot and is dependent
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on the success or failure of services. This way you can never tell
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exactly how and in which order they are executed. On multiprocessor
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machines, where processes may be truely executed in parallel, there
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is no order anymore, but there are n orders on n processors.
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To solve this issue, init systems which support parallel execution
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should ship with some testing tools, that simulate a startup.
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cinit will ship with such a tool in cinit-0.4.
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References
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----------
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no content yet
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