Extend disk for KVM virtual images

You have a KVM virtual machine which boots from a “disk” mapped to a file on the host system, and at some point you realize you need to extend that.

We assume the disk file is called /mnt/vm/05.qcow2 and we want to extend it.

• download GParted and copy it to the host system

• Power the VM off, take advantage of the downtime to also increase RAM and CPU, if needed. Make sure your VM has a virtual CD/DVD.

• On the host system, extend the disk-file:

  qemu-img resize /mnt/vm/05.qcow2 +50G
  # On our host, file extension is qcow2 but really the disk format is RAW
  root@pa1-r1-s06:~# qemu-img info /mnt/vm/05.qcow2
  image: /mnt/vm/05.qcow2
  file format: raw
  virtual size: 100G (107374182400 bytes)
  disk size: 48G
  

• from virt-manager, double-click on your VM name, which will pop-up the VM management window:

  • go to the settings tab, from Boot Options select enable boot menu

  • connect the CD/DVD-Rom to the GParted image

  • boot the VM, press F12 to enter boot menu and select boot from CD/DVD

  • GParted will start, extend /dev/vda to maximum size, apply changes, reboot (you will be prompted for disconnecting the GParted image from CD/DVD, and press Enter)

  • boot the machine normally, once it’s up connect as root and:

    • extend the LVM volume (we keep couple of GBs free):

      lvextend -L+48G /dev/vgroot/lvroot
      

    • resize the filesystem (it is ext4 in our case):

      resize2fs /dev/vgroot/lvroot
      

    • reboot

Verify the filesystem has grown by (nearly) 50G, enjoy!