Using KVM on CentOS

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Introduction

Having the ability to run a Kernel Basd Virtual Machine (KVM) on a hosted solution is a luxury you won’t easily find on most cloud providers (Amazon, Google, Azure). On my hosting provider, I can get a quad core i7 32gb RAM server for less than 70 euros / month, a fraction of the cost that you would pay with a cloud provider.

I tought it might be interesting to see what kind of virtualization options I had on Centos and stumbled upon KVM, LibVRT and Qemu.

Installing some software

yum -y install qemu-kvm libvirt virt-install bridge-utils
yum -y install kvm virt-manager libvirt virt-install qemu-kvm xauth dejavu-lgc-sans-fonts
yum -y install /usr/bin/virt-sysprep

Verify install

[root@CentOS-72-64-minimal ~]# lsmod | grep kvm 
kvm_intel             162153  12 
kvm                   525409  1 kvm_intel

Verify service

[root@CentOS-72-64-minimal ~]# systemctl status libvirtd
● libvirtd.service - Virtualization daemon
   Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; enabled; vendor preset: enabled)
   Active: active (running) since Sat 2017-03-18 13:32:02 CET; 8h ago
     Docs: man:libvirtd(8)
           http://libvirt.org
 Main PID: 11490 (libvirtd)
   Memory: 6.1G
   CGroup: /system.slice/libvirtd.service
           ├─11490 /usr/sbin/libvirtd
           ├─11563 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelper
           └─11564 /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelper

If the service isn’t started, start it using

systemctl status libvirtd

Creating a VM

You can create virtual machine with the following command

virt-install --name centos7 --ram 4096 \
--disk path=/var/kvm/images/centos7.img,size=30 \
--vcpus 2 --os-type linux --os-variant rhel7 \
--network bridge=virbr0 --graphics none --console pty,target_type=serial \
--location 'http://ftp.iij.ad.jp/pub/linux/centos/7/os/x86_64/' 
--extra-args 'console=ttyS0,115200n8 serial'

The command above will start the installation process. You can then continue the text based installation and let it boot. Aftr the installation you can shutdown the machine.

you can always return to the host by pressing CTRL-].

Executing virsh console centos7 will get you back into the VM.

Commands

Starting VMs

virsh start centos7
virsh start centos7 --console

Stopping VMs

virsh shutdown centos7 
virsh destroy centos7

Console VMs

virsh console centos7

Removing VMs

virsh undefine centos7

Cloning a VM

Now that you’ve done the entire installation of a VM, it might be interesting to make a clone of it.

Create a clone by issueing the following command

virt-clone --original centos7 --name template --file /var/kvm/images/template.img

This will generate a template.xml file and a tmplate image in the provided location.

Now that we have a template, it’s very easy to create some additional VMs basd on that template.

virt-clone --original template --name centos-a --file /var/kvm/images/centos-a.img
virt-clone --original template --name centos-b --file /var/kvm/images/centos-b.img
virt-clone --original template --name centos-manager --file /var/kvm/images/centos-manager.img

Importing an existing template

If you ever want to migrate VMs on another machine, its sufficient to copy the image file and the xml file, and execute the command below:

virsh define template.xml 

File storage

Where does all of this get stored ?

  • XML files are located in /etc/libvirt/qemu/
  • Image files are located in /var/kvm/images/

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