This is useful when you for example want to expand the disk space of your virtual machine when it is using LVM, or if you want to add a swap disk to a virtual machine.

Note that you can also create a swap file instead of a disk, however, this is an example for adding the disk.

• Host running KVM and virsh
• Virtual Machine to add disk to

The example VM is named example-vm in virsh (domain).

Execute these steps on the KVM hypervisor host.

cd to the folder where you store your disk images:

cd /var/lib/libvirt/images/

sudo qemu-img create -f raw example-vm-swap.img 1G

We use qemu-img to create a new raw disk image with a size of 1 GB.

virsh attach-disk example-vm --source /var/lib/libvirt/images/example-vm-swap.img --target vdb --persistent

We use virsh to attach the disk image /var/lib/libvirt/images/example-vm-swap as a virtio (/dev/vdb) disk to the domain (vm) example-vm.

The –persistent option updates the domain xml file with an element for the newly attached disk.

Note that if you already have a /dev/vdb disk you need to change vdb to a free device like vdc or vdd.

Reboot so that the kernel sees the new disk.

Execute this in the virtual machine.

sudo reboot

Partition the drive with cfdisk.

For our example we use filesystem type 82 (linux/linux swap):

Execute this in the virtual machine.

cfdisk /dev/vdb

Format the disk as swap:

mkswap /dev/vdb1

or format it as ext4:

mkfs.ext4 /dev/vdb1

swapon /dev/vdb1

or mount the partition:

mkdir /mnt/new-disk
mount /dev/vdb1 /mnt/new-disk

Add to /etc/fstab for reboot persistence:

/etc/fstab

/dev/vdb1   swap            swap    defaults    0 0

or for the ext4 disk:

/etc/fstab

/dev/vdb1   /mnt/new-disk   ext4    defaults    0 0

That's it. You've now created, attached, formatted and mounted a new disk in your VM.

• qemu-img man page [http://linux.die.net/man/1/qemu-img]
• virsh-attach doc page [http://builder.virt-tools.org/artifacts/libvirt-virshcmdref/html/sect-attach-disk.html]