debian stretch: upgrade 32-bit to 64-bit

Posted on 2017-06-28

There are various reasons why one would prefer 64-bit over 32-bit (or not); basically it is about improved ABI (passing arguments in registers), bigger register (can be faster) versus higher memory usage (because pointers are twice as big). In some corner cases you want 64-bit to be able to use more memory in your programs (32-bit kernels can often handle more than 4G memory, but 32-bit userspace programs can’t).

For me 64-bit offers higher security as well: ASLR (address space layout randomization) should be more effective on 64-bit because it can use more entropy.

So that is why I decided to upgrade some 32-bit (called “i386” in debian) machines to 64-bit (“amd64”); after some trial and error I came to the following set of operations to do this in a “reliable” (for me) and fast way.

Instructions

  • This one should be obvious: make sure you have backups
  • If you can’t figure out what one of the commands below does, this is not for you. Expert mode only :)
  • Don’t blame me if it fails :)
  • Some programs store data in a platform specific way. So far I had this problem only with postgresql, but other databases are very likely to be affected too. Export the data in a platform independent way before and restore it afterwards.
  • If you run custom installed stuff like rvm, virtualenv or npm they probably won’t work after the migration.
  • Remember which packages were installed, so you can later check nothing is missing (and you didn’t install packages you didn’t want). As I use aptitude with “markauto” I also store these states:

      aptitude search ~M | cut -c5- | cut -d' ' -f1 > aptitude.auto
      dpkg -l | grep '^ii' | awk '{print $2}' > packages
    
  • Check the current architectures. Should show “i386” in the first and either nothing or “amd64” (when multiarch is already enabled) in the second command.

      dpkg --print-architecture
      dpkg --print-foreign-architectures
    
  • Now upgrade to a 64-bit kernel. Since stretch you already need a multiarch setup for this:

      dpkg --add-architecture amd64
      apt-get update
      apt-get --no-install-recommends install linux-image-amd64:amd64
    

    If you need dkms modules you also need linux-headers-amd64:amd64. Make sure the dkms modules actually got compiled before rebooting if they are essential.

  • Reboot and make sure you boot the amd64 kernel - either manipulate your bootloader config before rebooting, select it manually, or remove the 32-bit kernel before.

      uname -r
      # should show 4.9.0-3-amd64
    
  • Now is a good time to remove special 32-bit only packages, like libc6-xen or the 32-bit kernel.

  • You’ll see a repeating pattern below, so I’ll explain it first. apt-get (and aptitude) are not good at resolving conflicts, and don’t understand they don’t have to remove the :i386 packages to install the corresponding :amd64 package. So they would start with removing the :i386 packages, which essentially breaks your system, instead of just installing the :amd64 package.

    So we only download the packages with apt, and install them manually with dpkg. For this we need a clean /var/cache/apt/archives/ directory.

    The pattern is like this:

      apt-get clean
      apt-get -y --no-install-recommends --download-only install SOME_PACKAGES
      dpkg --install /var/cache/apt/archives/*.deb
    

    If the download command shows errors, it usually mentions missing dependencies: just repeat the command and add those at the end of the command.

    Sometimes dpkg fails:

    • If a Pre-Depends package is missing, it won’t install it at all. At the end dpkg will show you the *.deb filename - just install those again with dpkg --install ....
    • If a Depends package is missing it won’t configure the package. Just run dpkg --configure -a afterwards to fix those.

    The examples below sometimes anticipate the dpkg failures and show a second command to run.

  • For whatever fucked up reason the dash preinst script uses bash; and bash of course “Pre-Depends” on dash. There is a good chance this breaks later, and if one of them gets removed you’re in big trouble (xxd -r might help you restoring binaries from another system). So we update these first:

      apt-get clean
      apt-get -y --no-install-recommends --download-only install dash:amd64 bash:amd64
      dpkg --install /var/cache/apt/archives/*.deb
      dpkg --install /var/cache/apt/archives/dash_*.deb /var/cache/apt/archives/bash_*.deb
    
  • Now we change the host architecture by installing the new packaging tools. This should work without aptitude too if you don’t want it.

      apt-get clean
      apt-get -y --no-install-recommends --download-only install dpkg:amd64 apt:amd64 aptitude:amd64 apt-utils:amd64
      dpkg --install /var/cache/apt/archives/*.deb
      dpkg --install /var/cache/apt/archives/dpkg_*.deb
    
  • Check the architecture indeed changed:

      dpkg --print-architecture
      dpkg --print-foreign-architectures
    
  • Now install all :i386 with :amd64 as well. You can start with the libraries and perl (this sometimes keeps the failing list shorter, but not always).

      apt-get clean
      apt-get --download-only -y --no-install-recommends install `dpkg -l | grep '^.i' | awk '{print $2}' | grep :i386 | sed -e 's/\(.*\):i386/\1:i386- \1:amd64/'`
      dpkg --install /var/cache/apt/archives/lib*.deb /var/cache/apt/archives/perl*.deb
      dpkg --configure -a
    

    The download command might fail due to missing dependencies (see note above); also packages which are not available on amd64 can’t be installed of course. You could try removing them if they are not strictly needed, or put a | grep -v PKGNAME before sed.

  • Now install everything else as :amd64.

      apt-get clean
      apt-get --download-only -y --no-install-recommends install `dpkg -l | grep '^.i' | awk '{print $2}' | grep :i386 | sed -e 's/\(.*\):i386/\1:i386- \1:amd64/'`
      dpkg --install /var/cache/apt/archives/*.deb
    

    Again you might need to fix the download command like before. Instead of redownloading you can just remove the just installed packages too, or just install them again.

    This was basically the big step.

  • Make sure your stuff is working again. The following steps cleanup the 32-bit packages, and for example installing postgresql-9.6:i386 to recover your database becomes more difficult if all dependecies are already gone.

  • Now remove the :i386 packages - they shouldn’t be needed anymore. I used aptitude for this (limit package view with l, then enter ~ri386). Something like this might work too:

      apt-get autoremove
      apt-get remove `dpkg -l | grep '^.i' | awk '{print $2}' | grep :i386`
    
  • Restore the aptitude markauto state:

      aptitude markauto `cat aptitude.auto | sed -e s/:i386/:amd64/`
    
  • Check installed package differences:

      diff -u  <(sed s/:.*// packages | uniq) <(dpkg -l | grep '^ii' | awk '{print $2}' | sed s/:.*// | uniq) | less
    

    Lines with + are new packages, lines with - are removed packages.

  • Remove i386 architecture:

      dpkg --remove-architecture i386
    

    Only works if there are no i386 packages left.

  • You’re done. Reboot if you want to make sure everything is still working :)

Additional notes

Debian multiarch dependency handling

Sometimes the dependency handling is fucked up. E.g. postgresql (architecture independent) depends on postgresql-9.6 - it isn’t multiarch enabled as far as I can tell, and this means it wants postgresql-9.6 with the host architecture (which is completely unreasonable as it is just a meta package pulling components).

A similar problem exists the other way too: postgresql-9.6 (architecture dependent) depends on locales (architecture independent). postgresql-9.6 wants locales to be from the same architecture as itself, and apt and dpkg pretend locales is from the host architecture, which doesn’t match if you try to install postgresql-9.6:i386 and the host architecture is already amd64.

I have no solution for the first problem; in the second case just download the package with wget and install it with dpkg --install. It won’t configure it (because a dependency is missing), but it should be usable.

Useful tools

  • needrestart is a great tool to find processes which should be restarted after upgrades - or 32-bit to 64-bit migrations.
  • etckeeper is a great tool to keep track of your /etc/ changes.
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