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Thread: Migrating a Xen DomU

  1. #1
    Join Date
    Jun 2008

    Default Migrating a Xen DomU

    I'd like to pick the brains of the Xen gurus out there.

    I have an Ubuntu DomU running on an Ubuntu Dom0 that I would like to migrate to a Centos Dom0. Both are x86_64.

    From what I can gather, I ought to be able to just tar up the domu (excluding the /sys /dev /proc parts) and then untar and 'xm create' it in the destination.

    And it looks like this might work fine, with the following minor adjustments:
    a.) edit the xen config file to change the kernel and ramdisk options from ubuntu to the centos equivalents
    b.) copying the centos xen modules to /lib/modules

    Problem is that when the domu boots in centos, it bails out at the point where it's supposed to mount the root fs. The physical disk devices (LV's) and mappings to the pseudo disk devices are identical as in ubuntu, as is the root= option in the config file.

    I've come to the conclusion that the problem is probably because the kernel cannot find the disk devices in /dev at boot time, so I need to get /dev in the domu populated, but can't figure out how to do this. Copying the /dev from the centos Dom0 doesn't work and chrooting the ubuntu domu and running the /sbin/MAKEDEV script doesn't seem to do anything either.

    Anyone know where I'm going wrong with this?

    From what I've managed to find on xen template creation, it seems like it should be dead easy, so I either don't understand it properly, or it's more complicated that it seems.

    While on the subject of templates, does anyone know a good source for information on how to create xen paravirt templates for different OS's?


  2. #2
    Join Date
    Jun 2008


    Well it turns out that in principle it is actually as easy as it looks.

    As usual, after hunting all day for a solution, I found the answer shortly after posting to a public forum (ralic's law of forums).

    A Xen Guest OS fails to boot with a "switchroot: mount failed: No such file or directory error message" error message.

    Generating the initrd without scsi modules did the trick for me.

  3. #3
    Join Date
    Dec 2007


    I don't have any answers off-hand, but I might recommend that you ask around the Xen Community (in addition to mailing lists there are several Xen groups on various social networking sites which may be able to assist).

    Update: Ralic's law appears to apply quite well

    I should also have mentioned that our SA's suggested it could be an initrd issue, though it sounds like you go that sorted anyhow.

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