mypapers/200802-bootsequence/200802-bootsequence.html

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  11 <a href="../200706-bootseq/200706-bootseq.html">last related talk</a>
  12
  13 <h1><A href="http://www.fosdem.org/2008/schedule/events/debian_boot">Reordering the Debian boot sequence for correctness and speed</a></h1>
  14
  15 <p>There are subtle bugs in the Debian boot and shutdown sequence.
  16 They are hard to find, as they normally only affect rare combination
  17 of packages, and harder to fix, as they normally require the combined
  18 work of several maintainers and changes in several packages.  This
  19 talk is about how we can solve them, and gain a few advantages on the
  20 way.</p>
  21
  22 <div class="presenter">Petter Reinholdtsen - one of the sysvinit maintainers
  23   <br>pere@hungry.com
  24   <br>FOSDEM 2008, 2008-02-26</div>
  25
  26 <!--
  27
  28 There are subtle bugs in the debian boot and shutdown sequence. They
  29 are hard to find, as they normally only affect rare combination of
  30 packages, and harder to fix, as they normally require the combined
  31 work of several maintainers and changes in several packages.
  32
  33 One way to find these bugs is to document the dependencies of all
  34 init.d scripts, and use this information to check that the ordering is
  35 correct. When such information is available, it is also possible to
  36 reorder the boot and shutdown sequence to make sure all dependencies
  37 are fulfilled.
  38
  39 It is also possible to run scripts in parallel, to speed up the boot,
  40 when the order they need to run in is known.
  41
  42 This talk is about how all of this can be done with Debian.
  43 -->
  44
  45 <h2>Summary</h2>
  46
  47 <!--
  48
  49  - how do sysvinit boot work
  50    - runlevels
  51    - inittab
  52    - rc*.d/
  53  - ordering problem
  54  - how does it work in debian
  55  - effect on the boot
  56  - insserv
  57  - speed change?
  58  - concurrency
  59
  60  - how to write lsb headers
  61    - provides
  62    - deps
  63    - runlevel list
  64
  65  - status in debian
  66    - need better doc
  67    - fix insserv bugs
  68
  69  - what is needed to convert
  70    - add LSB header to packages and get the change into testing
  71    - update policy
  72    - more users to test headers and detect errors
  73    - install and activate insserv by default
  74
  75 -->
  76
  77 <h2>SysV init in Debian - Booting</h2>
  78
  79 <p>Note, this is the stuff going on after the initrd part is done.
  80 The very early boot is done before hard drive partitions is mounted.</p>
  81
  82 <ol>
  83
  84 <li>/sbin/init start, which look at /etc/inittab and decides what to
  85   do.</li>
  86
  87 <li>The scripts in /etc/rcS.d/ is executed in sequence by
  88   /etc/init.d/rc, with <tt>start</tt> as the argument.</li>
  89
  90 <li>Depending on the runlevel, the scripts for the given runlevel is
  91   executed (normally the ones in /etc/rc2.d/) are executed in
  92   sequence, first the stop scripts with <tt>stop</tt> as their
  93   argument, next the start scripts with <tt>start</tt> as their
  94   argument.  The rc*.d/ directories contain symlinks the files in to
  95   /etc/init.d/.</li>
  96
  97 <li>The ordering is important.</li>
  98
  99 <li>The single-user runlevel will present a login prompt after rcS.d/
 100   was executed.  Runlevel 1 is not the single user runlevel, but it
 101   behaves as a better single user.</li>
 102
 103 </ol>
 104
 105 <h2>SysV init in Debian - Switching runlevels</h2>
 106
 107 <ul>
 108
 109 <li>Call <tt>telinit X</tt> where X is the new runlevel, one of S, 0,
 110   1, 2, 3, 4, 5, 6.</li>
 111
 112 <li>init runs in sequence all stop scripts in /etc/rcX.d/ that also
 113    has a start symlink in the previous runlevel /etc/rcY.d/, with
 114    <tt>stop</tt> as their argument.</li>
 115
 116 <li>init then run in sequence all start scripts in /etc/rcX.d/, with
 117   <tt>start</tt> as their argument.</li>
 118
 119 </ul>
 120
 121 <p>Note that switching to runlevel S will not run the scripts in
 122 /etc/rcS.d/.  To get a similar effect after boot, switch to runlevel
 123 1.  It will (should) kill all services and prepare the machine for
 124 maintenance.</p>
 125
 126 <h2>SysV init in Debian - Shutting down</h2>
 127
 128 <p>This is equivalent to switching to runlevel 0 (halt) or 6 (reboot),
 129   with a minor exception.  All scripts (both start and stop) are
 130   executed with the <tt>stop</tt> argument.
 131
 132 <h2>The ordering problem</h2>
 133
 134 <p>Script ordering is vital for this to work.  And how are the scripts
 135 ordered?  By numbers.
 136
 137 <p>And the numbers are picked using skills, knowledge and negotiation.
 138 Getting it right is often hard.
 139
 140 <p>The current Debian default is wrong.  Stop sequence should by
 141 default be the reverse of the start sequence.  It isn't.
 142
 143 <p>Reordering is hard and require cooperation between maintainers of
 144 all packages involved.  Given two packages with two scripts inserted
 145 with the default settings in Debian:
 146
 147 <p>Package A: script_a sequence 20 (start and stop)
 148 <br>Package B: script_b sequence 20 (start and stop)
 149
 150 <p>Along come script C, which should run before script_a and after
 151 script_b.  Current solution is to change packages A and C or packages
 152 B and C to get something like this:
 153
 154 <p>Package A: script_a start seq. 22, stop seq. 18
 155 <br>Package B: script_b sequence 20 (start and stop)
 156 <br>Package C: script_c start seq 21, stop seq 19
 157
 158 <p>If other scripts depend on the old order of script_a, they will
 159 have to change their sequence number too.  Only way to discover this
 160 is by a lot of testing, or documenting script dependencies.
 161
 162 <h2>A ordering solution</h2>
 163
 164 <p>Let each script document its dependency, and generate sequence
 165 numbers using this dependency information.  Example:
 166
 167 <p>Package A: script_a depend on nothing
 168 <br>Package B: script_b depend on nothing
 169 <br>Package C: script_c depend on script_b, a dependency of script_a
 170
 171 <p>Generated sequence:
 172
 173 <p>script_b start seq 1, stop seq 3
 174 <br>script_c start seq 2, stop seq 2
 175 <br>script_a start seq 3, stop seq 1
 176
 177 <p>An implementation of this system is the dependency based boot
 178 sequencing, provided in the insserv package.</p>
 179
 180
 181 <h2>LSB headers for insserv</h2>
 182
 183 <ul>
 184   <li>"Provides" header, list the Facility/service provided by the
 185     script.  Do not list virtual facilities (like $time)</li>
 186
 187   <li>Runlevel entries (Default-Start and Default-Stop headers), list
 188     what runlevels to activate for this script
 189
 190   <li>Dependency entries (Required-Start, Required-Stop, Should-Start,
 191     Should-Stop, X-Start-Before, X-Stop-After), list the
 192     facilities/services needed by this script.  It will start after
 193     its start dependencies and stop before its stop dependencies.  The
 194     X-* entreis are reverse dependencies.  Required-* are hard
 195     dependencies (will install even if they are missing), while
 196     Should-* and X-* are soft dependencies (only taken into account if
 197     scripts providing them are present).</li>
 198
 199 </ul>
 200
 201 <h2>What to list as dependencies I</h2>
 202
 203 <p>If your package used the default update-rc.d settings before, this
 204 is the header for you:</p>
 205 <pre>
 206 ### BEGIN INIT INFO
 207 # Provides:          scriptname
 208 # Required-Start:    $remote_fs $syslog
 209 # Required-Stop:     $remote_fs $syslog
 210 # Default-Start:     2 3 4 5
 211 # Default-Stop:      0 1 6
 212 ### END INIT INFO
 213 </pre>
 214
 215 <p>$remote_fs is needed by all scripts using files in /usr/.  $syslog
 216 is needed only by scripts starting services logging to syslog.</p>
 217
 218
 219 <h2>Status of dependency based boot</h2>
 220
 221 <img alt="LSB header progress graph" src="lsb-header-progress.png" width="50%" align="right">
 222
 223 <p>Release goal for Debian Lenny.
 224
 225 <p>Packages with LSB header: 654 of 866 (76%)
 226 <br>Unsolved BTS reports: XXX
 227 <br>Packages without BTS reports: ~150
 228
 229 <p>Need better documentation
 230
 231 <p>Need debian policy updates
 232
 233 <p>Fix insserv bugs
 234
 235 <p>
 236
 237     <h2>Thank you very much</h2>
 238
 239     <h3>Questions?</h3>
 240
 241     <p><a href="http://www.hungry.com/~pere/mypapers/200802-bootsequence/200802-bootsequence.html">http://www.hungry.com/~pere/mypapers/200802-bootsequence/200802-bootsequence.html</a></p>
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