KVM - Add Disk

I run my virtual machines on SSD storage which is expensive compared to traditional HDD storage. Thus if a virtual machine needs a lot of storage, such as for a mirror, then I will give it an HDD disk to put the data on. Here's how to do just that.

Create the Disk

Qcow2

qemu-img create \
  -f qcow2 \
  -o lazy_refcounts=on,preallocation=metadata \
  $FILEPATH \
  [size]G

Raw

qemu-img create \
  -f raw \
  $FILEPATH \
  [size]G

Attach the Disk

Qcow2

virsh attach-disk $VM_ID \
  --source /path/to/disk.qcow2 \
  --target vd[x] \
  --persistent \
  --subdriver qcow2

The path to the disk for source has to be an absolute path, not a relative one. The command can be run to attach the disk whilst the VM Is running.

Raw

virsh attach-disk $VM_ID \
  --source /path/to/disk.raw \
  --target vd[x] \
  --persistent \
  --subdriver raw

At this point the disk should show up in your VM if you run the command:

ls /dev/vd*

To use the disk we need to create a filesystem on it.

sudo mkfs -t ext4 /dev/vd[x]

Now we need a place to mount the disk and add it to our /etc/fstab so that it will mount on boot.

sudo mkdir /media/data

Then add the following to your fstab

/dev/vdb /media/data ext4 defaults 0 2

Now you can mount everything

sudo mount -a

Detach Disk

If you wish to remove a disk, you can do so like so:

sudo virsh detach-disk $GUEST_ID vd[x]