Setting Up LVM Backed Xen DomU

LVM volumes as backing for DomU’s file system is an appealing solution to Xen VBD. LVM volumes can dynamically grow/shrink and snapshot. These features make it simple and fast to duplicate DomU and adding storage to DomU. Creating LVM-backed Xen DomU is introduced in this post.

Create LVM-backed VBD

Suppose we have a LVM volume group for Xen now:

vg_xen. Let’s create a logical volume of size 20GB named ‘vm-10.0.0.123’ for the VM we will create:

# lvcreate -L20480 -n 'vm-10.0.0.123' vg_xen

Please refer to

Creating LVM Volume Group for setting up LVM volume group.

The profile for LVM-backed DomU

Then we can create a profile for LVM-bakced Xen DomU using the LVM logical volume we create above:

name="lvm-backed-domainu"
vcpus=2
memory=2048
disk=['phy:mapper/vg_xen-vm--10.0.0.123,xvda,w']
# disk=['phy:vg_xen/vm-10.0.0.123,xvda,w']
vif=['bridge=eth0']
bootloader="/usr/bin/pygrub"
on_reboot='restart'
on_crash='restart'

or for installing a new DomU:

name="F12INSTALL"
vcpus=2
memory=2048
disk=['phy:mapper/vg_xen-vm--10.0.0.123,xvda,w']
# disk=['phy:vg_xen/vm-10.0.0.123,xvda,w']
vif = [ 'bridge=eth0' ]
kernel = "/lhome/xen/f12install/vmlinuz"
ramdisk = "/lhome/xen/f12install/initrd.img"
on_reboot = 'restart'
on_crash = 'restart'

The virtual machine’s name is “10.0.0.123”, memory is 2G, CPU number is 2, VBD is LV /dev/mapper/vg_xen/vm-10.0.0.123. The “disk” line specifies that we will use logical volume

vm-10.0.0.123 in volume group vg_xen for this DomU’s VBD. Then we can install operating system in the DomU. Please refer to the “Install Fedora 12 Linux via Internet” section in: Setting Up Xen DomU on Fedora Linux (Fedora 12) for installing Fedora 12 in Xen DomU.

For more tutorials about Xen, please refer to Xen Solutions.

Eric Z Ma

Eric is a father and systems guy. Eric is interested in building high-performance and scalable distributed systems and related technologies. The views or opinions expressed here are solely Eric’s own and do not necessarily represent those of any third parties.

16 comments:

  1. Hi, Eric,

    My Volume Group is fedora_VG, and my logical volumn is vm1, I wite :

    disk=[‘phy:mapper/fedora_VG–vm1,xvda,w’]
    there is error as follow:

    libxl__device_disk_set_backend: Disk vdev=xvda failed to stat:mapper/fedoraVG–vm1:no such file or directory.

    how can I solve this problem ?

    Thank you

  2. disk=[‘phy:fedora_VG/vm1,xvda,w’]
    or
    disk=[‘phy:mapper/fedora_VG–vm1,xvda,w’]
    or
    disk=[‘/dev/mapper/fedora_VG-vm1,xvda,w’]
    or
    disk=[‘/dev/mapper/fedora_VG/vm1,xvda,w’]

    they are all error.

    How to solve this?
    Thank you ;-)

  3. NO. this one does not work
    disk=[‘phy:fedora_VG/vm1,xvda,w’]
    but if I modify it to :

    disk=[‘phy:fedora_VG/vm1, raw, xvda,w’]
    the config file is OK.

    but after that the install need a URL, I type this URL:
    http://archives.fedoraproject.org/pub/archive/fedora/linux/releases/12/Fedora/x86_64/os/

    the installation exit here.

    My lvs looks like this:

    LV VG ATTR LSize
    root fedora_VG -wi-ao 97G
    swap fedora_VG -wi-ao 3G
    vm1 fedora_VG -wi-a 20G

    I don’t know why attr of vm1 is only -wi-a, there is no ‘o’ here.

    1. But in the other comments, you show the one without phy: . Both are good?

      Which version of Linux and xen is the Dom0? The config file seems changed a lot. F12 is too old. You may consider a newer release if there is no specific reason to use F12.

  4. Hi, Eric,

    When I install Domain U, my config file is lvmdomu1.cfg, like this:

    name = ‘lvmdomU1’
    kernel = “/home/shhpeng/Xen-dev/vmlinuz”
    ramdisk = “/home/shhpeng/Xen-dev/initrd.img”
    disk = [‘/dev/mapper/fedora_VG-vm1, raw, xvda, rw’]
    extra = “root=/dev/xvda1”
    vif = [‘bridge=xenbr0’]
    memory = 512
    vcpus = 1
    maxvcpus = 1

    it works,

    But when I copy the config file to another file lvmdomu1.run, and modify it like this:

    name = ‘lvmdomU1-run’

    disk = [‘/dev/mapper/fedora_VG-vm1, raw, xvda, rw’]
    extra = “root=/dev/xvda1″
    bootloader=”pygrub”
    vif = [‘bridge=xenbr0’]
    memory = 512
    vcpus = 1
    maxvcpus = 1

    then

    xl create -f lvmdomu1.run -c

    there is error :
    Traceback (most recent call last):
    File “/usr/local/lib/xen/bin/pygrub”, line 20 , in
    import xen.lowlevel.xc
    ImportError: No module named xen.lowlevel.xc

    How can I solve this problem?
    Thank you.

  5. Hi, Eric,

    the file ” lvmdomu1.run” is the configuration file when I want to boot the domain U.

    The domain U has already been installed.

    Thank you.

  6. The problem is python:

    ImportError: No module named xen.lowlevel.xc

    I solve this by :
    #make dist PYTHON_PREFIX_ARG=
    #make install PYTHON_PREFIX_ARG=

  7. Hi, Eric
    There is another problem.

    When I boot VM, the following message will display :

    PCI : Fatal : No config space access function found.

    What is this ?

    Thank you.

  8. if I login in the VM using root and I cannot modify file “/boot/grub/grub.cfg”.

    it is read only.

    How can I modify this file ?

    Thank you .

    1. It is either the Dom0 or the DomU that makes your /boot/ read-only and there are many possible reasons.

      Without knowing details of your Dom0 and DomU, the others can’t figured out what’s wrong.

  9. Hi, Eric,

    I cann’t use the Internet on domU. How can I copy a file from dom0 to LVM Backed Xen DomU ?

    Thank you.

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