In this example we’ll use the default pool and a volume named vdisk-10G.qcow2. We’ll show how to create the volume from the host, how to format it with common filesystems (ext4, FAT32, XFS, btrfs) from inside the guest, and finally how to add 10 GB to the volume and resize the partition and filesystem.

Note: commands that manage volumes (create, resize) run on the host and use your prompt. Commands executed inside the guest (partitioning and formatting) are shown with the guest prompt javiercruces@debian13:~$.

1 Create the volume (host)

We’ll create the volume in qcow2 format (thin-provisioned). You can use virsh vol-create-as or qemu-img depending on your preference.

javiercruces@FJCD-PC:~$ # Create with libvirt in the `default` pool (qcow2, thin)
javiercruces@FJCD-PC:~$ virsh vol-create-as --pool default --format qcow2 vdisk-10G.qcow2 10G

javiercruces@FJCD-PC:~$ # Alternative: create the file directly with qemu-img
javiercruces@FJCD-PC:~$ qemu-img create -f qcow2 /var/lib/libvirt/images/vdisk-10G.qcow2 10G

Verify it exists:

javiercruces@FJCD-PC:~$ virsh vol-list default
 Name            Path
-------------------------------------------------
 vdisk-10G.qcow2 /var/lib/libvirt/images/vdisk-10G.qcow2

You can inspect the file with qemu-img info to confirm it’s qcow2 and check allocation:

javiercruces@FJCD-PC:~$ qemu-img info /var/lib/libvirt/images/vdisk-10G.qcow2
image: /var/lib/libvirt/images/vdisk-10G.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K

2 Attach the volume to a VM for formatting (host)

Attach the volume to a VM (e.g. testguest1) as vdb live and persistently:

javiercruces@FJCD-PC:~$ virsh attach-disk --live --config testguest1 /var/lib/libvirt/images/vdisk-10G.qcow2 vdb

3 Create a single partition and format inside the guest

Connect to the guest (example):

javiercruces@FJCD-PC:~$ ssh javiercruces@debian13
javiercruces@debian13:~$ sudo -i

Use parted to create a GPT (or msdos) partition table and a single partition that occupies the whole disk:

javiercruces@debian13:~$ sudo parted /dev/vdb --script mklabel gpt mkpart primary 0% 100%

After creating the partition the kernel typically exposes /dev/vdb1. Examples to format the partition with common filesystems:

  • ext4
javiercruces@debian13:~$ sudo mkfs.ext4 -F /dev/vdb1
sudo mkdir -p /mnt/vdb1 && sudo mount /dev/vdb1 /mnt/vdb1
sudo bash -c 'echo "ext4 test" > /mnt/vdb1/README.txt'
  • FAT32 (vfat)
javiercruces@debian13:~$ sudo apt update && sudo apt install -y dosfstools
javiercruces@debian13:~$ sudo mkfs.vfat -F 32 /dev/vdb1
sudo mkdir -p /mnt/vdb1 && sudo mount -o uid=1000,gid=1000 /dev/vdb1 /mnt/vdb1
sudo bash -c 'echo "FAT32 test" > /mnt/vdb1/README.txt'
  • XFS
javiercruces@debian13:~$ sudo apt update && sudo apt install -y xfsprogs
javiercruces@debian13:~$ sudo mkfs.xfs -f /dev/vdb1
sudo mkdir -p /mnt/vdb1 && sudo mount /dev/vdb1 /mnt/vdb1
sudo bash -c 'echo "XFS test" > /mnt/vdb1/README.txt'
  • Btrfs
javiercruces@debian13:~$ sudo apt update && sudo apt install -y btrfs-progs
javiercruces@debian13:~$ sudo mkfs.btrfs -f /dev/vdb1
sudo mkdir -p /mnt/vdb1 && sudo mount /dev/vdb1 /mnt/vdb1
sudo bash -c 'echo "BTRFS test" > /mnt/vdb1/README.txt'

Unmount when finished with testing:

javiercruces@debian13:~$ sudo umount /mnt/vdb1
exit

4 Extend the volume by 10 GB (host)

On the host increase the volume size to 20G (virsh vol-resize uses the absolute size):

javiercruces@FJCD-PC:~$ virsh vol-resize --pool default vdisk-10G.qcow2 20G

If the VM is using the disk live, the following in-guest steps will allow the partition and FS to use the new space.

5 Resize partition and filesystem inside the guest

Reconnect to the guest (or reuse an existing session). Use growpart when available to expand the partition, then use the filesystem-specific tool to grow the filesystem.

javiercruces@FJCD-PC:~$ ssh javiercruces@debian13
javiercruces@debian13:~$ sudo apt update && sudo apt install -y cloud-guest-utils
javiercruces@debian13:~$ sudo growpart /dev/vdb 1
  • ext4 (online)
javiercruces@debian13:~$ sudo resize2fs /dev/vdb1
  • XFS (online, must be mounted, grow on the mount point)
javiercruces@debian13:~$ sudo mount /dev/vdb1 /mnt/vdb1    # if not already mounted
javiercruces@debian13:~$ sudo xfs_growfs /mnt/vdb1
  • Btrfs (online)
javiercruces@debian13:~$ sudo mount /dev/vdb1 /mnt/vdb1
javiercruces@debian13:~$ sudo btrfs filesystem resize max /mnt/vdb1
  • FAT32

FAT32 is less flexible: to resize it online you may need tools like fatresize (not always available) or perform the operation offline from the host with guestfish or by unmounting and running fatresize inside the guest. Example if fatresize is available:

javiercruces@debian13:~$ sudo apt install -y fatresize
javiercruces@debian13:~$ sudo fatresize -s 20G /dev/vdb1

If fatresize is not available, the alternative is to perform the change offline (unmount and use guestfish or boot from a rescue ISO) or recreate the partition and restore data from backup.