(Last Updated On: April 19, 2019)

I personally use KVM for all Linux virtualization projects. Sometimes I need to extend or add disk space to my running VM (guest) to satisfy growing software requirements. KVM uses QEMU which supports several image types, among them raw, cow, qcow, qcow2, vmdk, vdi e.t.c.

The “native” and most flexible type is qcow2, which supports copy on write, encryption, compression, and VM snapshots.

Step 1: Shut down the VM

Before you can extend your guest machine Virtual disk, you need to first shut it down.

$ sudo virsh list
 Id   Name    State
-----------------------
 4    rhel8   running

If your guest machine is in running state, power it off using its ID or Name.

$ sudo virsh shutdown rhel8
Domain rhel8 is being shutdown

Confirm that it is truly down before proceeding to manage its disks.

$  sudo virsh list          
 Id   Name   State
--------------------

Step 2: Extend guest OS disk

Locate your guest OS disk path.

$ sudo virsh domblklist rhel8
 Target   Source
-----------------------------------------------
 vda      /var/lib/libvirt/images/rhel8.qcow2
 sda      -

OR use:

$ sudo virsh dumpxml rhel8 | egrep 'disk type' -A 5
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/rhel8.qcow2'/>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
--
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='sda' bus='sata'/>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

You can obtain the same information from the Virtual Machine Manager GUI. My VM disk is located in ‘/var/lib/libvirt/images/rhel8.qcow2‘.

$ sudo qemu-img info /var/lib/libvirt/images/rhel8.qcow2
image: /var/lib/libvirt/images/rhel8.qcow2
file format: qcow2
virtual size: 30G (42949672960 bytes)
disk size: 2.0G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: true
    refcount bits: 16
    corrupt: false

Step 3: Extend guest VM disk

Since we know the location of our Virtual Machine disk, let’s extend it to our desired capacity.

sudo qemu-img resize /var/lib/libvirt/images/rhel8.qcow2 +10G

Please note that qemu-img can’t resize an image which has snapshots. You will need to first remove all VM snapshots. See this example:

$ sudo virsh snapshot-list rhel8
 Name        Creation Time               State
--------------------------------------------------
 snapshot1   2019-04-16 08:54:24 +0300   shutoff

$ sudo virsh snapshot-delete --domain rhel8 --snapshotname snapshot1
Domain snapshot snapshot1 deleted

$ sudo virsh snapshot-list rhel8                                    
 Name   Creation Time   State
-------------------------------

Then extend the disk by using the `+‘ before disk capacity.

$ sudo qemu-img resize /var/lib/libvirt/images/rhel8.qcow2 +10G
Image resized.

You can also resize with virsh command. This requires domain to be running.

$ sudo qemu-img info /var/lib/libvirt/images/rhel8.qcow2
image: /var/lib/libvirt/images/rhel8.qcow2
file format: qcow2
virtual size: 30G (42949672960 bytes)
disk size: 2.0G
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: true
refcount bits: 16
corrupt: false

$ sudo virsh start rhel8
$ sudo virsh blockresize rhel8 /var/lib/libvirt/images/rhel8.qcow2 40G
Block device '/var/lib/libvirt/images/rhel8.qcow2' is resized

Confirm disk size with fdisk command.

$ sudo fdisk -l /var/lib/libvirt/images/rhel8.qcow2      
Disk /var/lib/libvirt/images/rhel8.qcow2: 30.2 GiB, 32399818752 bytes, 63280896 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Step 4: Grow VM partition

Now power up the VM

$ sudo virsh start rhel8
Domain rhel8 started

SSH to your VM as root user or using user account that has sudo.

$ ssh rhel8             
Last login: Fri Apr 19 06:11:19 2019 from 192.168.122.1
[[email protected] ~]$

Check your new disk layout.

$ lsblk 
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sr0 11:0 1 1024M 0 rom
vda 252:0 0 40G 0 disk
├─vda1 252:1 0 1G 0 part /boot
└─vda2 252:2 0 29G 0 part
├─rhel-root 253:0 0 26.9G 0 lvm /
└─rhel-swap 253:1 0 2.1G 0 lvm [SWAP]

My VM total disk capacity is now 40GB, previously it was 30GB. To extend your OS partition, refer to below guides.

How to extend root filesystem using LVM in Linux

How to resize an ext2/3/4 and XFS root partition without LVM

Other guides:

How to extend EBS boot disk on AWS without an instance reboot

How to create disk partitions in Windows using diskpart command

Working with qemu-img in Linux