Archive for category KVM

Video of my Livebackup talk at KVM Forum 2011

This is a quick post. Here is a link to the video of my Livebackup presentation at the KVM Forum 2011.

Here is a link to the presentation:


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kvm virtualization under Oracle Linux 6 or CentOS 6 (Clone of Redhat Linux 6)

Recently, I got around to installing Oracle Linux 6 (RHEL 6 clone) on a machine in order to experiment with kvm. The machine I installed it on is an Intel Core 2 Duo 6400 with 4 GB of RAM. Intel Virtualization Technology (VT) is present in this chip, and enabled in the BIOS. I am running the server headless.

At install time, I chose the ‘virtual host’ option.

Setting up a bridge ‘br0’ in order to enable VM bridged networking:

Oracle Linux installs a default bridge virbr0 that is useful if you want to configure the VM to use ‘host only’ networking. I wanted a bridged network VM, i.e. the VM’s virtual interface should appear on my physical network just as any other machine would. There are a few steps that I need to do in order to enable this:

  • Create a new bridge ‘br0’ and assign it the static IP address that used to be associated with eth0.
  • Make the physical network interface ‘eth0’ be an uplink port to this bridge ‘br0’.

The assumption here is that the physical network card in the system is ‘eth0’. If you have ‘eth1’ connected to the network, make the corresponding changes to the setup described. Another assumption here is that at the end of the Linux install, ‘eth0’ has the static IP address with gateway

First create a new file /etc/sysconfig/network-scripts/ifcfg-br0 with the following contents:


Next, delete the old ifcfg-eth0 file, and create a new one with the following contents:


Setup /etc/resolv.conf to point to the free google DNS Servers:


Reboot the system. When it comes up again,  the bridge br0 should have the IP address, and eth0 should be an uplink port on the bridge, as shown below:

br0       Link encap:Ethernet  HWaddr 00:1C:C0:07:20:70
inet addr:  Bcast:  Mask:
inet6 addr: fe80::21c:c0ff:fe07:2070/64 Scope:Link
RX packets:69422 errors:0 dropped:0 overruns:0 frame:0
TX packets:38962 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:8360996 (7.9 MiB)  TX bytes:9988797 (9.5 MiB)

eth0      Link encap:Ethernet  HWaddr 00:1C:C0:07:20:70
inet6 addr: fe80::21c:c0ff:fe07:2070/64 Scope:Link
RX packets:69433 errors:0 dropped:0 overruns:0 frame:0
TX packets:39489 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:9335186 (8.9 MiB)  TX bytes:10020863 (9.5 MiB)

Setting up yum to use a disk based copy of the install DVD as its repository:

After installation, I copied the contents of the install DVD into a directory called /root/cdrom. Then I ran the command ‘createrepo .’ from the /root/cdrom directory. Note that I had to do a ‘rpm –install’ of the createrepo rpm before I could do this. I also created a file /etc/yum.repos.d/iso.repo with the following contents:


One more step:

I ran the following on Oracle Linux:

# rpm --import /root/cdrom/RPM-GPG-KEY

I ran the following on CentOS 6:

# rpm --import /root/cdrom/RPM-GPG-KEY-CentOS-6

Now, yum can find rpms from the /root/cdrom directory.

Starting to install RHEL6 in a newly created blank VM using the libvirt command line tool virt-install:

First, I turned off the firewall using ‘/etc/init.d/iptables stop’, since I want to connect to the guest console using vncviewer from my desktop. Remember, this server is running headless.

# mkdir -p /vms/1

# virt-install --name=el6guest --arch=x86_64 --ram=512 --os-type=linux --os-variant=rhel6 --hvm --network bridge=br0 --cdrom=/dev/cdrom --disk path=/vms/1/vdisk0,size=16 --accelerate --vnc --vnclisten=

Starting install...
Creating storage file vdisk0                                                                          |  16 GB     00:00
Creating domain...                                                                                    |    0 B     00:00
Cannot open display:
Run 'virt-viewer --help' to see a full list of available command line options
Domain installation still in progress. You can reconnect to
the console to complete the installation process.

Connecting to the console of the newly created VM in order to start installation:

The VM el6guest has now been created by virt-install, but we dont yet know which vnc port the guest is listening on. Run the command virsh as follows:

[root@localhost ~]# virsh vncdisplay el6guest

The ‘:0’ printed out by virsh tells us that the vnc server for guest el6guest is listening on port 5900, i.e. display 0

Startup the vncviewer binary on your desktop, and connect to the VM server at display 0. The Oracle Linux installer console will come up in vncviewer.

I installed a base server with root password el6guest. Once installation is complete, hit restart. The VM will shutdown at this point. You can restart the VM from virsh as shown below:

[root@localhost qemu]# virsh
Welcome to virsh, the virtualization interactive terminal.
Type:  'help' for help with commands
'quit' to quit
virsh # start el6guest
Domain el6guest started
virsh # vncdisplay el6guest

Now, you can connect to display 0 using a vncviewer from your desktop, and the VM’s console will show up. That’s all folks..

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Mounting partitions of a virtual disk image in Linux

Often, you may need to mount individual partitions of a virtual disk image. Here is a cheat sheet for doing so under Linux:

The first sector of a hard disk contains the Master Boot Record or MBR. In the case of a virtual disk image file, the first 512 bytes of the file represent the MBR.

The disk is divided into partitions, and each partition can be formatted with a different filesystem. In order to mount the filesystem, you need to locate the partition table entry, determine the first sector of the partition, and supply that to the mount command for mounting.

The partition table is located at offset 0x1be (decimal 446). To dump the first partition entry type the following command:

# od -A d -t x1 vdisk0

Locate offset 446 of the print out:

0000432 00 00 00 00 00 00 00 00 55 20 06 00 00 00 80 01
0000448 01 00 83 fe ff 0e 3f 00 00 00 10 f0 bf 00 00 00

The first partition table entry is highlighted in red above. Bytes 9,10,11 and 12 constitute the offset, in sectors, of the beginning of the partition. It is highlighted in green above. Remember that it is in Little Endian byte order, so the 32 bit number is actually 0x0000003f, i.e. 63 decimal. This is in sector count, and each sector is 512 bytes long, so the actual file offset of the beginning of the first partition in file vdisk0 is (63 * 512) i.e. 32256.

Hence, use the following command to mount the first partition of virtual disk file vdisk0

# mount -o loop,offset=32256 ./vdisk0 ./mnt

Now you can ‘cd’ into ‘./mnt’ and view and modify files in the first partition of vdisk0. Remember to umount ./mnt when you are done.

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Creating a KVM VM with virtio disk and net drivers in CentOS/RedHat 5.4

I set out to accomplish a (seemingly) simple task: Install CentOS 5.4 with KVM Virtualization on a system and then create a CentOS 5.4 KVM VM with virtio Net and Disk drivers.

It turns out that there is more to this task than meets the eye. So, here’s my step by step procedure.

Step 1: Install Centos (Redhat 5.4) with KVM Virtualization on a Intel VT or AMD Pacifica enabled server (I used a Intel Core 2 Duo E6420/2GB/120GB SATA system)

  • Install CentOS 5.4 64 bit with the “Virtualization” option
  • While installing, choose the “Customize now” instead of customize later, and select KVM instead of ‘Virtualization’ in the Virtualization customization screen.
  • For this install, I chose to disable SELinux. I’m sure its useful in some security contexts, but for most of my use – it is just a source of endless problems. Someday, I might actually spend the time to learn how SELinux works. Right now, it feels to me like the Windowsification of Linux. Moving on…
  • When the newly installed system boots up, you need to create a bridge(software switch) called br0, move the IP address of eth0 to br0, and then make eth0 an uplink to the bridge br0. Here’s how to do it:
    • Create a file /etc/sysconfig/network-scripts/ifcfg-br0 with the following contents:
      • DEVICE=br0
    • Edit /etc/sysconfig/network-scripts/ifcfg-eth0 and replace its contents with:
      • DEVICE=eth0
    • Reboot your system. Note that this configuration is for a static IP server.
    • Add the following lines to /etc/sysconfig/iptables to allow relevant traffic:
      • -A RH-Firewall-1-INPUT -i br0 -j ACCEPT
      • -A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 5900:6900 -j ACCEPT

Step 2: Create a CentOS 5.4 KVM VM using the CentOS boot CD

Well, this task got complicated quickly. My intent was to make this VM connect to a bridged network interface, so that I could benchmark it by running nuttcp to another physical machine. CentOS 5.4 (Redhat 5.4) does not come with scripts for a bridged VM network out of the box. This is why we needed to create the br0 bridge in the previous step.

  • In order to use the br0 bridge effectively, we need a utility called tunctl (I have a precompiled version here – ). Please this in /usr/sbin on your new CentOS box.
  • It turns out that creating a VM by calling qemu directly involves a lot of options. I ended up building a script for this purpose. You can download it here: Place in /usr/sbin.
  • Create a directory for our VM called /vms/1
  • Create two 8GB files vdisk0 and vdisk1 in this directory using the following commands:
    • dd if=/dev/zero of=./vdisk0 count=1 bs=1 seek=8589934591
    • dd if=/dev/zero of=./vdisk1 count=1 bs=1 seek=8589934591
  • Create a file called vm.params with the following contents ( a sample is available at
    • MAC=52:54:00:00:00:01
  • For booting and installing the VM from the CentOS 5.4 CD image, run the following command:
    • /usr/sbin/ /vms/1/vm.params /tmp/CentOS-5.4-i386-bin-DVD.iso boot_from_cd
  • This will cause a tap interface called tap0 to be created, connected to the bridge br0, and the VM created by calling kvm-qemu directly
  • The script sets up the VM to publish a graphical console using the VNC protocol at TCP port 5900 + $VNCDISP, where VNCDISP is set in the vm.params file. Use your favorite vncviewer to connect to this graphical console.
  • When the VM is started up, you will get the graphical console of the VM. Now go through the process of installing the OS on your newly created VM

Step 3: First boot of the CentOS in your newly created VM

After the OS installation is completed, you can reboot the VM from the virtual hard drive, without the CDROM image attached. Here is the command to do that:

/usr/sbin/ /vms/1/vm.params

There you have it – a KVM VM with paravirtualized drivers (virtio) for network and disk.

All this without the use of libvirt or virt-manager or one of the myriad programs that did not quite work for me.

Step 4: Fixup VNC mouse tracking

One of the most annoying things about the qemu vnc server is the fact that the mouse works like cr**. Here’s a simple fix for that problem. Download the following xorg.conf file and place it in your newly created VM’s /etc/X11 directory. This configures a VNC screen of size 1024×768 with a mouse that actually works –

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