KVM (Kernel Virtualization Module) is an open source, popular and efficient virtualization technology provided by Linux kernel. Virtualization creates virtual ram, devices, disks, CPU’s, networks etc. Qemu is the user space tool used to provide these to the kernel level virtualization tool KVM. Disk is one of the most important device in the virtual systems. Qemu-img is the tool used to create, manage, convert shrink etc. the disk images of virtual machines.

Help

We can get help about the qemu-img command with the -h option.

$ qemu-img -h

Syntax

Syntax of the qemu-img command is like below.

qemu-img create -f fmt -o options fname size

Create Disk Image

Virtual Machines or simply VM’s runs operating system and user level tools like normal PC. Operating system and user level tools are stored in disk image files like physical disks. Generally all VM’s uses one disk image file to read and write data. We will use create command to create a disk image. In the example the disk image name will be ubuntu.img . We will provide disk size after disk name which is 10G in gigabyte.

$ qemu-img create ubuntu.img 10G

After disk image create some information is provided. The disk image format is raw and size is expressed as byte. We will look image formats below. Disk image actual size is 0 because there is no data in it but the vm will see disk image as 10G disk and will be able to use up to 10G.

Disk Image Types

As stated before qemu supports different type of disk image formats. Disk image format is different than file systems. Disk image format is related with the host system.

Raw

Raw is default format if no specific format is specified while creating disk images. Raw disk images do not have special features like compression, snapshot etc. Best thing about raw disk image is performance. Raw disk images are faster than other disk image types.

Qcow2

Qcow2 is opensource format developed against Vmdk and Vdi. Qcow2 provides features like compression, snapshot, backing file etc. It is popular in the Kvm, Qemu community.

Qed

Qed  is a disk format provided by Qemu. It provides support for overlay and sparse images. Performance of Qed is better than Qcow2 .

Qcow

Qcow is predecessor of the Qcow2.

Vmdk

Vmdk is default and popular disk image format developed and user by VMware. Vmware products like Player, Workstation, ESXi, uses vmdk disk image type. It is rival of Qcow2 format

Vdi

Vdi is popular format developed Virtual Box. It has similar features to the Vmdk and Qcow2

Vpc

Vps is format used by first generation Microsoft Virtualization tool named Virtual PC. It is not actively developed right now.

Create Qcow2 Disk Image

We can create qcow image with the -f option. We will also provide the disk size as an option. As you remember in the first example we have specified the disk size at the and of the command. Now we will set disk size with -o size option. In the example we create a qcow2 formatted disk image named ubuntu.img with the size of 10GB  .

$ qemu-img create -f qcow2 -o size=10G ubuntu.img

Create VMDK Disk Image

In this example we will create an vmdk disk image. We will set disk file name as ubuntu.vmdk . But the extension vmdk do not have an effect on the disk image type. We will also set disk image size 20GB with -o size option.

$ qemu-img create -f vmdk -o size=20G ubuntu.img

Get Information About Disk Image

Information about a disk file can be needed. As we learned previously the extension is not a sole disk image type shower. To get detailed information we will use info command by providing disk image name. In the example we will list information about ubuntu.vmdk

$ qemu-img info  ubuntu.img

• image shows the disk image file name
• virtual size shows the disk size of the image which will be read by VM
• disk size show the real size which hold in the host file system. It is 16K because there is no data on the disk image file

Shrink Disk Image

After some usage disk image file sizes grows. Even we delete files in the vm the size in the host system remains the same. Because the host system do not know how much and which part of the disk image file is unused. Shrink is done with convert command and copying existing disk image in to new one. In the example we will shrink disk image file named ubuntu.qcow2 into ubuntu_s.qcow2 .

$ qemu-img convert -O qcow2  ubuntu.qcow2 ubuntu_s.qcow2

Compress Disk Image

Another useful feature of the qemu-img is disk compression. Compressing disk images will make more space for other VMs. But keep in mind compression will little performance loss. We will use convert command to compress. In the example we will compress disk image file ubuntu.qcow2 into new file named ubuntu_c.qcow2 with -c option. This may take some time according to disk image file size and disk performance of the host system.

$ qemu-img convert -O qcow2 -c  ubuntu.qcow2 ubuntu_c.qcow2

Check Disk Image For Errors

Sometime while working with VM’s and disk images there can be errors. With qemu-img command these errors can be fixed. We will use check command for this operation. In this example we will provide disk image file name with the check command. Disk image format can be omitted because in most situations it is detected automatically.

$ qemu-img check ubuntu.qcow2

Increase Disk Image Size

While creating disk image files we generally give small amount of disk. But this may become problem in the feature as unexpectedly. We may need to increase the disk image file size. We will use resize command in order to increase disk size.In the example we will add extra 5GB to the disk image file named ubuntu.qcow2 .

$ qemu-img resize ubuntu.qcow2 +5GB

We can see from screenshot that new size is 25GB