+
The Debian installer could be
+a lot quicker. When we install more than 2000 packages in
+Skolelinux / Debian Edu using
+tasksel in the installer, unpacking the binary packages take forever.
+A part of the slow I/O issue was discussed in
+bug #613428 about too
+much file system sync-ing done by dpkg, which is the package
+responsible for unpacking the binary packages. Other parts (like code
+executed by postinst scripts) might also sync to disk during
+installation. All this sync-ing to disk do not really make sense to
+me. If the machine crash half-way through, I start over, I do not try
+to salvage the half installed system. So the failure sync-ing is
+supposed to protect against, hardware or system crash, is not really
+relevant while the installer is running.
+
+
A few days ago, I thought of a way to get rid of all the file
+system sync()-ing in a fairly non-intrusive way, without the need to
+change the code in several packages. The idea is not new, but I have
+not heard anyone propose the approach using dpkg-divert before. It
+depend on the small and clever package
+eatmydata, which
+uses LD_PRELOAD to replace the system functions for syncing data to
+disk with functions doing nothing, thus allowing programs to live
+dangerous while speeding up disk I/O significantly. Instead of
+modifying the implementation of dpkg, apt and tasksel (which are the
+packages responsible for selecting, fetching and installing packages),
+it occurred to me that we could just divert the programs away, replace
+them with a simple shell wrapper calling
+"eatmydata $program $@", to get the same effect.
+Two days ago I decided to test the idea, and wrapped up a simple
+implementation for the Debian Edu udeb.
+
+
The effect was stunning. In my first test it reduced the running
+time of the pkgsel step (installing tasks) from 64 to less than 44
+minutes (20 minutes shaved off the installation) on an old Dell
+Latitude D505 machine. I am not quite sure what the optimised time
+would have been, as I messed up the testing a bit, causing the debconf
+priority to get low enough for two questions to pop up during
+installation. As soon as I saw the questions I moved the installation
+along, but do not know how long the question were holding up the
+installation. I did some more measurements using Debian Edu Jessie,
+and got these results. The time measured is the time stamp in
+/var/log/syslog between the "pkgsel: starting tasksel" and the
+"pkgsel: finishing up" lines, if you want to do the same measurement
+yourself. In Debian Edu, the tasksel dialog do not show up, and the
+timing thus do not depend on how quickly the user handle the tasksel
+dialog.
+
+
+
+
+| Machine/setup |
+Original tasksel |
+Optimised tasksel |
+Reduction |
+
+
+
+| Latitude D505 Main+LTSP LXDE |
+64 min (07:46-08:50) |
+<44 min (11:27-12:11) |
+>20 min 18% |
+
+
+
+| Latitude D505 Roaming LXDE |
+57 min (08:48-09:45) |
+34 min (07:43-08:17) |
+23 min 40% |
+
+
+
+| Latitude D505 Minimal |
+22 min (10:37-10:59) |
+11 min (11:16-11:27) |
+11 min 50% |
+
+
+
+| Thinkpad X200 Minimal |
+6 min (08:19-08:25) |
+4 min (08:04-08:08) |
+2 min 33% |
+
+
+
+| Thinkpad X200 Roaming KDE |
+19 min (09:21-09:40) |
+15 min (10:25-10:40) |
+4 min 21% |
+
+
+
+
+
The test is done using a netinst ISO on a USB stick, so some of the
+time is spent downloading packages. The connection to the Internet
+was 100Mbit/s during testing, so downloading should not be a
+significant factor in the measurement. Download typically took a few
+seconds to a few minutes, depending on the amount of packages being
+installed.
+
+
The speedup is implemented by using two hooks in
+Debian
+Installer, the pre-pkgsel.d hook to set up the diverts, and the
+finish-install.d hook to remove the divert at the end of the
+installation. I picked the pre-pkgsel.d hook instead of the
+post-base-installer.d hook because I test using an ISO without the
+eatmydata package included, and the post-base-installer.d hook in
+Debian Edu can only operate on packages included in the ISO. The
+negative effect of this is that I am unable to activate this
+optimization for the kernel installation step in d-i. If the code is
+moved to the post-base-installer.d hook, the speedup would be larger
+for the entire installation.
+
+
I've implemented this in the
+debian-edu-install
+git repository, and plan to provide the optimization as part of the
+Debian Edu installation. If you want to test this yourself, you can
+create two files in the installer (or in an udeb). One shell script
+need do go into /usr/lib/pre-pkgsel.d/, with content like this:
+
+
+#!/bin/sh
+set -e
+. /usr/share/debconf/confmodule
+info() {
+ logger -t my-pkgsel "info: $*"
+}
+error() {
+ logger -t my-pkgsel "error: $*"
+}
+override_install() {
+ apt-install eatmydata || true
+ if [ -x /target/usr/bin/eatmydata ] ; then
+ for bin in dpkg apt-get aptitude tasksel ; do
+ file=/usr/bin/$bin
+ # Test that the file exist and have not been diverted already.
+ if [ -f /target$file ] ; then
+ info "diverting $file using eatmydata"
+ printf "#!/bin/sh\neatmydata $bin.distrib \"\$@\"\n" \
+ > /target$file.edu
+ chmod 755 /target$file.edu
+ in-target dpkg-divert --package debian-edu-config \
+ --rename --quiet --add $file
+ ln -sf ./$bin.edu /target$file
+ else
+ error "unable to divert $file, as it is missing."
+ fi
+ done
+ else
+ error "unable to find /usr/bin/eatmydata after installing the eatmydata pacage"
+ fi
+}
+
+override_install
+
+
+
To clean up, another shell script should go into
+/usr/lib/finish-install.d/ with code like this:
+
+
+#! /bin/sh -e
+. /usr/share/debconf/confmodule
+error() {
+ logger -t my-finish-install "error: $@"
+}
+remove_install_override() {
+ for bin in dpkg apt-get aptitude tasksel ; do
+ file=/usr/bin/$bin
+ if [ -x /target$file.edu ] ; then
+ rm /target$file
+ in-target dpkg-divert --package debian-edu-config \
+ --rename --quiet --remove $file
+ rm /target$file.edu
+ else
+ error "Missing divert for $file."
+ fi
+ done
+ sync # Flush file buffers before continuing
+}
+
+remove_install_override
+
+
+
In Debian Edu, I placed both code fragments in a separate script
+edu-eatmydata-install and call it from the pre-pkgsel.d and
+finish-install.d scripts.
+
+
By now you might ask if this change should get into the normal
+Debian installer too? I suspect it should, but am not sure the
+current debian-installer coordinators find it useful enough. It also
+depend on the side effects of the change. I'm not aware of any, but I
+guess we will see if the change is safe after some more testing.
+Perhaps there is some package in Debian depending on sync() and
+fsync() having effect? Perhaps it should go into its own udeb, to
+allow those of us wanting to enable it to do so without affecting
+everyone.
+
+
Update 2014-09-24: Since a few days ago, enabling this optimization
+will break installation of all programs using gnutls because of
+bug #702711. An updated
+eatmydata package in Debian will solve it.
+
+
Update 2014-10-17: The bug mentioned above is fixed in testing and
+the optimization work again. And I have discovered that the
+dpkg-divert trick is not really needed and implemented a slightly
+simpler approach as part of the debian-edu-install package. See
+tools/edu-eatmydata-install in the source package.
+
+
Update 2014-11-11: Unfortunately, a new
+bug #765738 in eatmydata only
+triggering on i386 made it into testing, and broke this installation
+optimization again. If unblock
+request 768893 is accepted, it should be working again.
+
+
