qemu-img

QEMU disk image operations. Health check, refcount repair, format conversion, and the NFS-to-local recovery pattern.

Disk Health Check

Check qcow2 image for errors — non-destructive read-only scan

sudo qemu-img check /var/lib/libvirt/images/9800-WLC-02.qcow2

Expected healthy output

No errors were found on the image.
76849/262144 = 29.32% allocated, 2.88% fragmented, 0.00% compressed clusters
Image end offset: 17180000256

Error output — refcount corruption (from NFS instability)

ERROR cluster 66221 refcount=0 reference=1
...
66 errors were found on the image.
Data may be corrupted, or further writes to the image may corrupt it.

If the VM is running, qemu-img check fails with Failed to get shared "write" lock. Shut down the VM first: sudo virsh destroy <vm-name>

Repair

Repair all refcount errors — fixes leaked and corrupted clusters

sudo qemu-img check -r all /var/lib/libvirt/images/9800-WLC-02.qcow2

Successful repair output

Repairing cluster 7970 refcount=1 reference=0
...
The following inconsistencies were found and repaired:
    10 leaked clusters
    0 corruptions

Double checking the fixed image now...
No errors were found on the image.

Never repair images on NFS — I/O errors during repair make it worse. Copy to local storage first, then repair.

Recovery pattern — copy from NFS, repair locally, update VM config

# 1. Stop the VM
sudo virsh destroy 9800-WLC-02

# 2. Copy from NFS to local storage
sudo cp /mnt/nas/vms/9800-WLC-02.qcow2 /var/lib/libvirt/images/9800-WLC-02.qcow2

# 3. Repair on local disk
sudo qemu-img check -r all /var/lib/libvirt/images/9800-WLC-02.qcow2

# 4. Update VM to use local copy
sudo virt-xml 9800-WLC-02 --edit --disk path=/var/lib/libvirt/images/9800-WLC-02.qcow2

# 5. Start VM
sudo virsh start 9800-WLC-02

Image Information

Show image format, size, allocation

qemu-img info /var/lib/libvirt/images/9800-WLC-02.qcow2

Image Creation

Create a new qcow2 image — thin provisioned (grows on demand)

qemu-img create -f qcow2 /var/lib/libvirt/images/newvm.qcow2 50G

Create a backing file image — linked clone for fast lab VMs

qemu-img create -f qcow2 -b /var/lib/libvirt/images/base.qcow2 -F qcow2 /var/lib/libvirt/images/linked-clone.qcow2

Image Conversion

Convert raw to qcow2

qemu-img convert -f raw -O qcow2 disk.raw disk.qcow2

Convert vmdk (VMware) to qcow2

qemu-img convert -f vmdk -O qcow2 disk.vmdk disk.qcow2