Installing a CentOS 5 Xen VM on a CentOS 4 Xen host system

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While the Xen VM infrastructure for Grid gateways was initially deployed with CentOS 4, CentOS 5 has become available in the meantime. Some parts of the grid infrastructure, notably SRB, will only install on a CentOS 5 system. Hence, there is a need to run a CentOS 5 system as a VM within the existing CentOS 4 infrastructure.

Contents

[edit] Preparing the bootstrap script

  • I have modified the bootstrapvm script to accept an additional 3rd parameter, target distribution name - where the value of CentOS-5 triggers special handling. If not specified, the argument defaults to CentOS-4. Usage:
    ./bootstrapvm /mnt/vmRoot/ gridgwtest CentOS-5
  • The bootstrapvm script now picks a yum.conf script depending on the distribution - either vmstrap/yum.conf for CentOS-4 or vmstrap/yum.conf-$DISTRO for a different distribution specified on the command-line.
  • Yum no longer uses local eceupdates repository - it was broken for CentOS-5, so we use monashupdates for both.
  • Note: a CentOS-5 (5.2) DVD must be mounted on /mnt/CentOS-Media
  • The script imports the correct distribution-specific RPM GPG keys.
  • The script uses /sbin/MAKEDEV instead of /dev/MAKEDEV (does not get created on CentOS-5) and executes MAKEDEV inside a chroot-ed environment (so that MAKEDEV links against the right libraries).

[edit] Necessary workarounds

  • Hack 1: CentOS-5 /bin/cp and /bin/touch segfault when run in CentOS-4 environment.
    • Solution: PAUSE (Ctrl-Z) yum right after installing coreutils, replace /bin/{cp,touch} with EL4 (keep as /bin/{cp.EL5,touch.EL5} )
    • Note: does not seem to be necessary anymore with the latest updates.
  • Hack 2: some program seem to fail when /proc and /sys are not mounted.
    • Solution (either when pausing for Hack 1 or before installation): 
mkdir -p /mnt/vmRoot/{proc,sys}
mount -t proc none /mnt/vmRoot/proc
mount -t sysfs none /mnt/vmRoot/sys
  • Hack 3: Don't use ECE updates, use monash updates instead (something broken on ECE updates breaks RPM dependencies)

[edit] Running the installation

  • Start the installation with 
./bootstrapvm /mnt/vmRoot/ gridgwtest CentOS-5

[edit] Installing CentOS-5 XenU kernel on host system

The host system must have the Linux kernel for the host in order to start the virtual machine. But, the CentOS 5 kernel-xen package clashes with a number of packages installed on the CentOS-4 host system.

Hence, manually copy /boot/vmlinuz and /lib/modules/`uname -r`/ from the CentOS-5 guest to the CentOS-4 host.

Afterwards, create a ramdisk and create a version-independent symlink for both the ramdisk and the vmlinuz image: 

mkinitrd --preload xenblk --preload xennet --omit-scsi-modules /boot/initrd-2.6.18-92.el5xen-xendrv.img 2.6.18-92.el5xen initrd-2.6.18-92.el5xen-xendrv.img
ln -s initrd-2.6.18-92.el5xen-xendrv.img initrd-2.6.18-el5xen-xendrv.img
ln -s vmlinuz-2.6.18-92.el5xen vmlinuz-2.6.18-el5xen

When creating the xen domain configuration file for the virtual machine, use these CentOS-5 kernel and ramdisk symlinks: 

kernel = "/boot/vmlinuz-2.6.18-el5xen"
ramdisk = "/boot/initrd-2.6.18-el5xen-xendrv.img"

That should complete the install - do a xm create -c <DomainName> and keep your fingers crossed.

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