Migration Guide

This page addresses migration from older to newer CentOS versions, and from other Enterprise Linux distributions based on the same set of upstream source packages to CentOS.

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• General considerations regarding migration:

• A fresh install is generally strongly preferred over an upgrade.

  1. Be sure you have a good (bootable or restorable) backup before doing anything major to a system you care about.
  2. It is often helpful to have a bootable Linux system on a separate set of partitions or better yet a separate disk. This can serve as a fall-back or rescue system, as well as an on-line backup. Disk space is cheap compared to sysadmin time.

  3. Migrations from other systems based on the same set of upstream source packages should be relatively straight-forward. See FAQ/CentOS3 Question #5 "How do I migrate a system from WBEL-3 (White Box Enterprise Linux) to CentOS 3?" for example.

  4. You may want to upgrade the operating system parts of the filesystem and leave user directories, such as /home in place. If you do leave the old user home directories in place expect some problems with the GNOME/KDE menus and other desktop items. It may be preferable to create new user accounts and mount the old directories somewhere else in the filesystem for reference, copying, or linking user's material after the migration.

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• If you decide to do an upgrade anyway...
  1. Back up anything you care about.

  2. Remember - A fresh install is generally strongly preferred over an upgrade.

  3. Clean up as much as possible before the upgrade, removing any RPMS you do not need, or that you know will be obsoleted.
  4. Do an anaconda upgrade. Yum upgrades, or "live" upgrades with other package managers, MAY be possible but are often more problematic - particularly with regard to glibc and kernel/drivers.
  5. Upgrades from the immediately preceding version are more likely to be successful than upgrades across multiple versions. For example, if you are running (the now 'end of life') CentOS 4 and want to upgrade to CentOS 6, first upgrade to the intermediate Major release of CentOS 5.

  6. Upgrades from systems other than the latest CentOS (WhiteBox, RHEL, Scientific Linux, TaoLinux, ...) may be possible but will also require more work cleaning up afterwards. Consider migrating to the corresponding CentOS release before upgrading.

  7. After upgrading use rpm -qa --last, preferably redirected to a file or piped to less, to find packages that were not updated. Left-over packages will be at the end of the output and will have dates earlier than the date of the upgrade.

  8. Remove any left-over packages that are not needed.
  9. If you find left-over packages that have version numbers greater than or equal to the current-release CentOS packages, use "rpm -Uvh --oldpackage foo-1.2-3.i386.rpm ... " (or possibly the ugly "rpm -Uvh --force ..." in a pinch) to get CentOS versions of packages that may have had lower or identical version numbers to replace the obsolete packages.

  10. Find/build CentOS x (where x=2,3,4,5,6) versions of remaining required/desired packages. ATrpms, RPMforge (Dag/Dries), and EPEL (Fedora Extras for EL) Repositories all have ELx packages that work with the corresponding CentOS versions. KaranbirSingh rebuilds Fedora Extras and some other packages for CentOS. Things that can't be found there can often be rebuilt from the Fedora Project (also formerly called Fedora Core, or Fedora Extras) src.rpm packages, or perhaps even a Fedora Project binary might used directly if you are feeling lucky (not recommended). Best to start with the packages at the Fedora/RH level from which the corresponding EL distro was forked. For CentOS 5 this was an Fedora Core 6 test version; for CentOS 6, Fedora 12 era.

  11. If older packages do not have versions that work with the current OS version, consider using virtualization to support them by running older OS versions under the current release. Viable options include VMware and Xen.

  12. Always beware when using/mixing 3rd-party repos. The yum protectbase or priorities plugins are your friends. Although both can be used together, the priorities plugin provides more functionality and using protectbase is unnecessary if the priorities plugin is configured properly. See PackageManagement/Yum/ProtectBase and PackageManagement/Yum/Priorities.

  13. Good Luck. Enjoy!

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• If upgrading, consider first "cloning" the system and upgrading the clone.
• As root, create the new partitions as desired - root, boot, home, swap - assume for example /dev/hde1 ,2, 3, 4...
• Make file systems

  # mke2fs -j -L /clone_boot /dev/hdb1
  # mke2fs -j -L /clone /dev/hde2
  # mkswap -L SWAP-hde3 /dev/hde3
  # mke2fs -j -L /clone_home /dev/hde5
  # mke2fs -j -L /clone_data /dev/hde6

• Make a mountpoint for the clone, say /clone

  # mkdir /clone

• Mount the partition that is to be the new root on /clone, make other mount points, mount all new partitions

  # mount /dev/hde2 /clone
  # mkdir /clone/boot /clone/home /clone/data
  # mount /dev/hde1 /clone/boot
  # mount /dev/hde5 /clone/home
  # mount /dev/hde6 /clone/data

• Do the clone operation

  # tar clf - /boot | tar xf - -C /clone
  # tar clf - /home | tar xf - -C /clone
  # tar clf - / | tar xf - -C /clone

• Edit /etc/fstab and /boot/grub/grub.conf adding entries for the clone partitions and boot stanzas

• Edit /clone/etc/fstab and /clone/boot/grub/grub.conf adding entries for the original partitions and boot stanzas

• Test-boot both systems before proceeding.
• Perform the upgrade on the clone system and write grub to the boot partition rather than the MBR
• Boot the original OS and update the grub boot stanza for the new OS - formerly clone.
• Boot the new OS. Optionally, make the new OS grub the default (preferably after testing)

  # grub-install /dev/hda (or /dev/sda)

• An admin may add stanzas to /boot/grub/grub/conf for other operating systems including the original pre-clone system, MS Windows, Ubuntu, FreeBSD, ...

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1. Migration an existing system from RHEL5 to CentOS 5

• "All that you should need to do is install centos-release, remove redhat-release rpms and just yum update the machine, which should bring in all packages changed by CentOS ( since they will have a slightly higher E-V-R )." - per KaranbirSingh on centos-list

  • You may also wish to remove the "rhnplugin" from yum (not necessary but gives an error msg)

• OR - per Craig White on centos-list Mon, 29 Oct 2007 (updated for 5.8)...

  # cp /etc/redhat-release /etc/redhat-release-saved
  # rpm -e --nodeps redhat-release-notes redhat-release yum-rhn-plugin redhat-logos
  # rpm -ivh \
  http://mirror.centos.org/centos/5/os/i386/CentOS/centos-release-5-8.el5.centos.i386.rpm  \
  http://mirror.centos.org/centos/5/os/i386/CentOS/centos-release-notes-5.8-0.i386.rpm \
  http://mirror.centos.org/centos/5/os/i386/CentOS/redhat-logos-4.9.99-11.el5.centos.noarch.rpm
  # yum update

• Sometime the rpm install using a URL may be problematic. In that case use wget or lftpget to get local copies of the files and install with "rpm -ivh ...".

Note: It is only possible to pull the latest 'point release' packages directory from the host: mirror.centos.org Older point release packages are relocated to: vault.centos.org and so for the foregoing example, we would use:

# cp /etc/redhat-release /etc/redhat-release-saved
# rpm -e --nodeps redhat-release-notes redhat-release yum-rhn-plugin redhat-logos
# rpm -ivh \
http://vault.centos.org/5.0/os/i386/CentOS/centos-release-5-0.0.el5.centos.2.i386.rpm  \
http://vault.centos.org/5.0/os/i386/CentOS/centos-release-notes-5.0.0-2.i386.rpm

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2. Migrate an existing system from RHEL6 or SL6 to CentOS 6

To migrate from RHEL6 to CentOS 6 fetch the latest versions of the following packages for your architecture and put them in a temporary directory. Rather than fetching packages with wget, as shown below, the CentOS-6 installation media can be mounted and packages installed from it. For CentOS 6.2 x86_64

mkdir TMP
yum remove rhnlib abrt-plugin-bugzilla redhat-release-notes*
rpm -e --nodeps redhat-release redhat-indexhtml
cd TMP
wget http://mirror.centos.org/centos/6/os/x86_64/Packages/centos-release-6-2.el6.centos.7.x86_64.rpm
wget http://mirror.centos.org/centos/6/os/x86_64/Packages/centos-indexhtml-6-1.el6.centos.noarch.rpm
wget http://mirror.centos.org/centos/6/os/x86_64/Packages/yum-3.2.29-22.el6.centos.noarch.rpm
wget http://mirror.centos.org/centos/6/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-10.el6.noarch.rpm
rpm -Uvh *.rpm
cd ..
rm -rf TMP
yum clean all
yum upgrade

For CentOS 6.2 i386 only the wget lines change:

wget http://mirror.centos.org/centos/6/os/i386/Packages/centos-release-6-2.el6.centos.7.i686.rpm
wget http://mirror.centos.org/centos/6/os/i386/Packages/centos-indexhtml-6-1.el6.centos.noarch.rpm
wget http://mirror.centos.org/centos/6/os/i386/Packages/yum-3.2.29-22.el6.centos.noarch.rpm
wget http://mirror.centos.org/centos/6/os/i386/Packages/yum-plugin-fastestmirror-1.1.30-10.el6.noarch.rpm

To migrate from Scientific Linux 6 the only change is that the "rpm -e" line becomes

rpm -e --nodeps sl-release sl-indexhtml

Additional changes for SL - The following will find some SL-specific packages:

rpm -qa | grep ^sl-
rpm -qa | grep SL_

These can be removed by (for instance):

rpm -e --nodeps sl-bookmarks sl-release-notes yum-conf-sl6x SL_desktop_tweaks

One might want to download and add a few more look-and-feel items, or load them from the CentOS DVD:

rpm -Uvh redhat-bookmarks-6-1.el6.centos.noarch.rpm redhat-logos-60.0.14-10.el6.centos.noarch.rpm \
  redhat-menus-14.0.0-3.el6.noarch.rpm redhat-rpm-config-9.0.3-34.el6.noarch.rpm

After the above, and updating, then you may want to see what additional SL packages you have:

rpm -qa --qf "%{VENDOR} \n" | sort | uniq

will show all the vendor tags for installed packages. Then

rpm -qa --qf "%{NAME} %{VENDOR} \n" | grep "Scientific Linux" | cut -d ' ' -f 1 | sort

will show remaining SL packages. It is probably not strictly necessary, but you could "purify" the system by

yum reinstall $(rpm -qa --qf "%{NAME} %{VENDOR} \n" | grep "Scientific Linux" | cut -d ' ' -f 1 | sort)

You may find some packages do not have CentOS equivalents as SL ships packages that CentOS does not. Repeat the previous rpm query to see what is left over.

Procedures should be similar for other closely-related distributions.

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• See also

  • MigratingTwoToFive

  • MigratingThreeToFive

  • MigratingFourToFive

  • MigratingFiveToSix

  • Alternate approach to Disk Cloning in Linux