blog/Speeding_up_the_Debian_installer_using_eatmydata_and_dpkg_divert.html

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  14  <h1>
  15      <a href="http://people.skolelinux.org/pere/blog/">Petter Reinholdtsen</a>
  16
  17  </h1>
  18
  19 </div>
  20
  21
  22     <div class="entry">
  23       <div class="title">Speeding up the Debian installer using eatmydata and dpkg-divert</div>
  24       <div class="date">16th September 2014</div>
  25       <div class="body"><p>The <a href="https://www.debian.org/">Debian</a> installer could be
  26 a lot quicker.  When we install more than 2000 packages in
  27 <a href="http://www.skolelinux.org/">Skolelinux / Debian Edu</a> using
  28 tasksel in the installer, unpacking the binary packages take forever.
  29 A part of the slow I/O issue was discussed in
  30 <a href="https://bugs.debian.org/613428">bug #613428</a> about too
  31 much file system sync-ing done by dpkg, which is the package
  32 responsible for unpacking the binary packages.  Other parts (like code
  33 executed by postinst scripts) might also sync to disk during
  34 installation.  All this sync-ing to disk do not really make sense to
  35 me.  If the machine crash half-way through, I start over, I do not try
  36 to salvage the half installed system.  So the failure sync-ing is
  37 supposed to protect against, hardware or system crash, is not really
  38 relevant while the installer is running.</p>
  39
  40 <p>A few days ago, I thought of a way to get rid of all the file
  41 system sync()-ing in a fairly non-intrusive way, without the need to
  42 change the code in several packages.  The idea is not new, but I have
  43 not heard anyone propose the approach using dpkg-divert before.  It
  44 depend on the small and clever package
  45 <a href="https://packages.qa.debian.org/eatmydata">eatmydata</a>, which
  46 uses LD_PRELOAD to replace the system functions for syncing data to
  47 disk with functions doing nothing, thus allowing programs to live
  48 dangerous while speeding up disk I/O significantly.  Instead of
  49 modifying the implementation of dpkg, apt and tasksel (which are the
  50 packages responsible for selecting, fetching and installing packages),
  51 it occurred to me that we could just divert the programs away, replace
  52 them with a simple shell wrapper calling
  53 "eatmydata&nbsp;$program&nbsp;$@", to get the same effect.
  54 Two days ago I decided to test the idea, and wrapped up a simple
  55 implementation for the Debian Edu udeb.</p>
  56
  57 <p>The effect was stunning.  In my first test it reduced the running
  58 time of the pkgsel step (installing tasks) from 64 to less than 44
  59 minutes (20 minutes shaved off the installation) on an old Dell
  60 Latitude D505 machine.  I am not quite sure what the optimised time
  61 would have been, as I messed up the testing a bit, causing the debconf
  62 priority to get low enough for two questions to pop up during
  63 installation.  As soon as I saw the questions I moved the installation
  64 along, but do not know how long the question were holding up the
  65 installation.  I did some more measurements using Debian Edu Jessie,
  66 and got these results.  The time measured is the time stamp in
  67 /var/log/syslog between the "pkgsel: starting tasksel" and the
  68 "pkgsel: finishing up" lines, if you want to do the same measurement
  69 yourself.  In Debian Edu, the tasksel dialog do not show up, and the
  70 timing thus do not depend on how quickly the user handle the tasksel
  71 dialog.</p>
  72
  73 <p><table>
  74
  75 <tr>
  76 <th>Machine/setup</th>
  77 <th>Original tasksel</th>
  78 <th>Optimised tasksel</th>
  79 <th>Reduction</th>
  80 </tr>
  81
  82 <tr>
  83 <td>Latitude D505 Main+LTSP LXDE</td>
  84 <td>64 min (07:46-08:50)</td>
  85 <td><44 min (11:27-12:11)</td>
  86 <td>>20 min 18%</td>
  87 </tr>
  88
  89 <tr>
  90 <td>Latitude D505 Roaming LXDE</td>
  91 <td>57 min (08:48-09:45)</td>
  92 <td>34 min (07:43-08:17)</td>
  93 <td>23 min 40%</td>
  94 </tr>
  95
  96 <tr>
  97 <td>Latitude D505 Minimal</td>
  98 <td>22 min (10:37-10:59)</td>
  99 <td>11 min (11:16-11:27)</td>
 100 <td>11 min 50%</td>
 101 </tr>
 102
 103 <tr>
 104 <td>Thinkpad X200 Minimal</td>
 105 <td>6 min (08:19-08:25)</td>
 106 <td>4 min (08:04-08:08)</td>
 107 <td>2 min 33%</td>
 108 </tr>
 109
 110 <tr>
 111 <td>Thinkpad X200 Roaming KDE</td>
 112 <td>19 min (09:21-09:40)</td>
 113 <td>15 min (10:25-10:40)</td>
 114 <td>4 min 21%</td>
 115 </tr>
 116
 117 </table></p>
 118
 119 <p>The test is done using a netinst ISO on a USB stick, so some of the
 120 time is spent downloading packages.  The connection to the Internet
 121 was 100Mbit/s during testing, so downloading should not be a
 122 significant factor in the measurement.  Download typically took a few
 123 seconds to a few minutes, depending on the amount of packages being
 124 installed.</p>
 125
 126 <p>The speedup is implemented by using two hooks in
 127 <a href="https://www.debian.org/devel/debian-installer/">Debian
 128 Installer</a>, the pre-pkgsel.d hook to set up the diverts, and the
 129 finish-install.d hook to remove the divert at the end of the
 130 installation.  I picked the pre-pkgsel.d hook instead of the
 131 post-base-installer.d hook because I test using an ISO without the
 132 eatmydata package included, and the post-base-installer.d hook in
 133 Debian Edu can only operate on packages included in the ISO.  The
 134 negative effect of this is that I am unable to activate this
 135 optimization for the kernel installation step in d-i.  If the code is
 136 moved to the post-base-installer.d hook, the speedup would be larger
 137 for the entire installation.</p>
 138
 139 <p>I've implemented this in the
 140 <a href="https://packages.qa.debian.org/debian-edu-install">debian-edu-install</a>
 141 git repository, and plan to provide the optimization as part of the
 142 Debian Edu installation.  If you want to test this yourself, you can
 143 create two files in the installer (or in an udeb).  One shell script
 144 need do go into /usr/lib/pre-pkgsel.d/, with content like this:</p>
 145
 146 <p><blockquote><pre>
 147 #!/bin/sh
 148 set -e
 149 . /usr/share/debconf/confmodule
 150 info() {
 151     logger -t my-pkgsel "info: $*"
 152 }
 153 error() {
 154     logger -t my-pkgsel "error: $*"
 155 }
 156 override_install() {
 157     apt-install eatmydata || true
 158     if [ -x /target/usr/bin/eatmydata ] ; then
 159         for bin in dpkg apt-get aptitude tasksel ; do
 160             file=/usr/bin/$bin
 161             # Test that the file exist and have not been diverted already.
 162             if [ -f /target$file ] ; then
 163                 info "diverting $file using eatmydata"
 164                 printf "#!/bin/sh\neatmydata $bin.distrib \"\$@\"\n" \
 165                     > /target$file.edu
 166                 chmod 755 /target$file.edu
 167                 in-target dpkg-divert --package debian-edu-config \
 168                     --rename --quiet --add $file
 169                 ln -sf ./$bin.edu /target$file
 170             else
 171                 error "unable to divert $file, as it is missing."
 172             fi
 173         done
 174     else
 175         error "unable to find /usr/bin/eatmydata after installing the eatmydata pacage"
 176     fi
 177 }
 178
 179 override_install
 180 </pre></blockquote></p>
 181
 182 <p>To clean up, another shell script should go into
 183 /usr/lib/finish-install.d/ with code like this:
 184
 185 <p><blockquote><pre>
 186 #! /bin/sh -e
 187 . /usr/share/debconf/confmodule
 188 error() {
 189     logger -t my-finish-install "error: $@"
 190 }
 191 remove_install_override() {
 192     for bin in dpkg apt-get aptitude tasksel ; do
 193         file=/usr/bin/$bin
 194         if [ -x /target$file.edu ] ; then
 195             rm /target$file
 196             in-target dpkg-divert --package debian-edu-config \
 197                 --rename --quiet --remove $file
 198             rm /target$file.edu
 199         else
 200             error "Missing divert for $file."
 201         fi
 202     done
 203     sync # Flush file buffers before continuing
 204 }
 205
 206 remove_install_override
 207 </pre></blockquote></p>
 208
 209 <p>In Debian Edu, I placed both code fragments in a separate script
 210 edu-eatmydata-install and call it from the pre-pkgsel.d and
 211 finish-install.d scripts.</p>
 212
 213 <p>By now you might ask if this change should get into the normal
 214 Debian installer too?  I suspect it should, but am not sure the
 215 current debian-installer coordinators find it useful enough.  It also
 216 depend on the side effects of the change.  I'm not aware of any, but I
 217 guess we will see if the change is safe after some more testing.
 218 Perhaps there is some package in Debian depending on sync() and
 219 fsync() having effect?  Perhaps it should go into its own udeb, to
 220 allow those of us wanting to enable it to do so without affecting
 221 everyone.</p>
 222
 223 <p>Update 2014-09-24: Since a few days ago, enabling this optimization
 224 will break installation of all programs using gnutls because of
 225 <a href="https://bugs.debian.org/702711">bug #702711</a>.  An updated
 226 eatmydata package in Debian will solve it.</p>
 227
 228 <p>Update 2014-10-17: The bug mentioned above is fixed in testing and
 229 the optimization work again.  And I have discovered that the
 230 dpkg-divert trick is not really needed and implemented a slightly
 231 simpler approach as part of the debian-edu-install package.  See
 232 tools/edu-eatmydata-install in the source package.</p>
 233
 234 <p>Update 2014-11-11: Unfortunately, a new
 235 <a href="http://bugs.debian.org/765738">bug #765738</a> in eatmydata only
 236 triggering on i386 made it into testing, and broke this installation
 237 optimization again.  If <a href="http://bugs.debian.org/768893">unblock
 238 request 768893</a> is accepted, it should be working again.</p>
 239 </div>
 240
 241       <div class="tags">Tags: <a href="http://people.skolelinux.org/pere/blog/tags/debian">debian</a>, <a href="http://people.skolelinux.org/pere/blog/tags/debian edu">debian edu</a>, <a href="http://people.skolelinux.org/pere/blog/tags/english">english</a>.</div>
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 506  <li><a href="http://people.skolelinux.org/pere/blog/tags/offentlig innsyn">offentlig innsyn (11)</a></li>
 507
 508  <li><a href="http://people.skolelinux.org/pere/blog/tags/open311">open311 (2)</a></li>
 509
 510  <li><a href="http://people.skolelinux.org/pere/blog/tags/opphavsrett">opphavsrett (49)</a></li>
 511
 512  <li><a href="http://people.skolelinux.org/pere/blog/tags/personvern">personvern (76)</a></li>
 513
 514  <li><a href="http://people.skolelinux.org/pere/blog/tags/raid">raid (1)</a></li>
 515
 516  <li><a href="http://people.skolelinux.org/pere/blog/tags/reactos">reactos (1)</a></li>
 517
 518  <li><a href="http://people.skolelinux.org/pere/blog/tags/reprap">reprap (11)</a></li>
 519
 520  <li><a href="http://people.skolelinux.org/pere/blog/tags/rfid">rfid (3)</a></li>
 521
 522  <li><a href="http://people.skolelinux.org/pere/blog/tags/robot">robot (9)</a></li>
 523
 524  <li><a href="http://people.skolelinux.org/pere/blog/tags/rss">rss (1)</a></li>
 525
 526  <li><a href="http://people.skolelinux.org/pere/blog/tags/ruter">ruter (4)</a></li>
 527
 528  <li><a href="http://people.skolelinux.org/pere/blog/tags/scraperwiki">scraperwiki (2)</a></li>
 529
 530  <li><a href="http://people.skolelinux.org/pere/blog/tags/sikkerhet">sikkerhet (41)</a></li>
 531
 532  <li><a href="http://people.skolelinux.org/pere/blog/tags/sitesummary">sitesummary (4)</a></li>
 533
 534  <li><a href="http://people.skolelinux.org/pere/blog/tags/skepsis">skepsis (4)</a></li>
 535
 536  <li><a href="http://people.skolelinux.org/pere/blog/tags/standard">standard (46)</a></li>
 537
 538  <li><a href="http://people.skolelinux.org/pere/blog/tags/stavekontroll">stavekontroll (3)</a></li>
 539
 540  <li><a href="http://people.skolelinux.org/pere/blog/tags/stortinget">stortinget (9)</a></li>
 541
 542  <li><a href="http://people.skolelinux.org/pere/blog/tags/surveillance">surveillance (27)</a></li>
 543
 544  <li><a href="http://people.skolelinux.org/pere/blog/tags/sysadmin">sysadmin (2)</a></li>
 545
 546  <li><a href="http://people.skolelinux.org/pere/blog/tags/valg">valg (8)</a></li>
 547
 548  <li><a href="http://people.skolelinux.org/pere/blog/tags/video">video (45)</a></li>
 549
 550  <li><a href="http://people.skolelinux.org/pere/blog/tags/vitenskap">vitenskap (4)</a></li>
 551
 552  <li><a href="http://people.skolelinux.org/pere/blog/tags/web">web (34)</a></li>
 553
 554 </ul>
 555
 556
 557     </div>
 558     <p style="text-align: right">
 559  Created by <a href="http://steve.org.uk/Software/chronicle">Chronicle v4.6</a>
 560 </p>
 561
 562   </body>
 563 </html>