increase a KVM guest's disk space
October 17, 2018 - Reading time: 2 minutes
  1. stop the VM
  2. run qemu-img resize vmdisk.img +10G to increase image size by 10Gb
  3. start the VM, resize the partitions and LVM structure within it normally

Online Method (using qemu, libvirt, and virtio-block)

For better or worse, the commands below will run even if the target virtual disk is mounted. This can be useful in environments where the disk cannot be unmounted (such as a root partition), the VM must stay on, and the system owner is willing to assume the risk of data corruption. To remove that risk, you would need to log into the VM and unmount the target disk first, something that isn't always possible.

Perform the following from the KVM hypervisor.

  1. Increase the size of the disk image file itself (specify the amount to increase):

    qemu-img resize <my_vm>.img +10G

  2. Get the name of the virtio device, via the libvirt shell (drive-virtio-disk0 in this example):

    virsh qemu-monitor-command <my_vm> info block --hmp
  drive-virtio-disk0: removable=0 io-status=ok file=/var/lib/libvirt/images/<my_vm>.img ro=0 drv=raw encrypted=0
  drive-ide0-1-0: removable=1 locked=0 tray-open=0 io-status=ok [not inserted]

  3. Signal the virtio driver to detect the new size (specify the total new capacity):

    virsh qemu-monitor-command <my_vm> block_resize drive-virtio-disk0 20G --hmp

Then log into the VM. Running dmesg should report that the virtio disk detected a capacity change. At this point, go ahead and resize your partitions and LVM structure as needed.

how to mount qcow2 image
August 14, 2018 - Reading time: ~1 minute

Step 1 - Enable NBD on the host

modprobe nbd max_part=8

Step 2 - Connect the QCOW2 as a network block device

qemu-nbd --connect=/dev/nbd0 /var/lib/vz/images/100/vm-100-disk-1.qcow2

Step 3 - List partitions inside the QCOW2

fdisk /dev/nbd0 -l

Step 4 - Mount the partition from the VM

mount /dev/nbd0p1 /mnt/somepoint/

You can also mount the filesystem with normal user permissions, ie. non-root:

mount /dev/nbd0p1 /mnt/somepoint -o uid=$UID,gid=$(id -g)

Step 5 - After you're done, unmount and disconnect

umount /mnt/somepoint/
qemu-nbd --disconnect /dev/nbd0
rmmod nbd

converting between image formats
January 10, 2018 - Reading time: 9 minutes

qemu-img convert: raw, qcow2, qed, vdi, vmdk, vhd

The qemu-img convert command can do conversion between multiple formats, including qcow2, qed, raw, vdi, vhd, and vmdk.

Installing qemu-img:

[ ~]# yum install qemu-img
qemu-img format strings

Image format

Argument to qemu-img

QCOW2 (KVM, Xen)

qcow2

QED (KVM)

qed

raw

raw

VDI (VirtualBox)

vdi

VHD (Hyper-V)

vpc

VMDK (VMware)

vmdk

This example will convert a raw image file named image.img to a qcow2 image file.

$ qemu-img convert -f raw -O qcow2 image.img image.qcow2

Run the following command to convert a vmdk image file to a raw image file.

$ qemu-img convert -f vmdk -O raw image.vmdk image.img

Run the following command to convert a vmdk image file to a qcow2 image file.

$ qemu-img convert -f vmdk -O qcow2 image.vmdk image.qcow2
 
Note

The -f format flag is optional. If omitted, qemu-img will try to infer the image format.

When converting an image file with Windows, ensure the virtio driver is installed. Otherwise, you will get a blue screen when launching the image due to lack of the virtio driver. Another option is to set the image properties as below when you update the image in the Image service to avoid this issue, but it will reduce virtual machine performance significantly.

$ openstack image set --property hw_disk_bus='ide' image_name_or_id

VBoxManage: VDI (VirtualBox) to raw

If you’ve created a VDI image using VirtualBox, you can convert it to raw format using the VBoxManage command-line tool that ships with VirtualBox. On Mac OS X, and Linux, VirtualBox stores images by default in the ~/VirtualBox VMs/ directory. The following example creates a raw image in the current directory from a VirtualBox VDI image.

$ VBoxManage clonehd ~/VirtualBox\ VMs/image.vdi image.img --format raw

Converting qcow2 image to raw image format using qemu-img:
[ ~]# qemu-img convert -f qcow2 -O raw image.qcow2 image.img

Converting raw image to qcow2 image format using qemu-img:

[ ~]# qemu-img convert -f raw -O qcow2 image.img image.qcow2 


























    

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