Working with qemu-img in Linux: This is a brief qemu-img cheatsheet for working with qemu-img command on Linux and Unix systems supporting qemu. I made this qemu-img cheatsheet for my own reference and saw a need to share with you guys. Before working with the qemu-img command to perform any disk operation, It’s good to first understand what qemu is and how it is used in Linux and Virtualization world.
What is Qemu?
According to the Qemu about page, QEMU is defined as a generic and open source machine emulator and virtualizer. This means that Qemu can be used as machine emulator to run Operating systems and programs for one machine on a different machine. An example is running ARM program on x86 PC. In this case, QEMU can use other hypervisors like Xen or KVM for CPU extensions, to achieve what is commonly referred to as Hardware Assisted Virtualization
When QEMU is used as a virtualizer, it executes the guest code directly on the host machine CPU to achieve near narrative performances. QEMU image formats are supported by most VPS cloud provides like Amazon, Digital Ocean, Linode, OVH and many others.
What is qemu-img?
qemu-img is the command line utility that’s used to convert various file systems used by hypervisors like Xen, KVM, VMware, VirtualBox. qemu-img is used to format guest images, add additional storage devices and network storage e.t.c. The output image format can be imported to VPS cloud providers like Openstack and other providers like Cloudstack. This working with qemu-img cheatsheet will show you a couple examples.
How to Install qemu-img in Linux?
The qemu-img command is installed by default with the libvirt stack:
How to install KVM on RHEL/CentOS 8, Fedora, Arch Linux, CentOS, Ubuntu/Debian, SLES
Commonly used qemu-img command options
The qemu-img uses the following general syntax:
qemu-img subcommand [options]The following list shows common subcommands for working with qemu-img:
| Subcommand | Description |
|---|---|
| create | Used to create a new disk image on the file system. |
| check | Used to check an existing disk image for errors. |
| convert | Used to converts an existing disk image from one format to another. |
| info | Used to display information about a disk image. |
| snapshot | Used to manage snapshots of an existing disk image. |
| commit | Use this to apply changes made to an existing disk image. |
| rebase | Used to create a new base image based on an existing disk image. |
| resize | Used to increase or decrease the size of an existing disk image. |
Common options
| Option | Explanation |
|---|---|
| – O | output format |
| -f | specify disk format |
Supported disk formats
The following image formats are supported :
| Format | Description |
|---|---|
| raw | This is the default image format. Can be exported to all other emulators. |
| qcow2 | QEMU image format which is most versatile format. |
| vmdk | VMware compatible image format |
| cloop | Useful only to reuse directly compressed CD-ROM images present e.g in Knoppix CD-ROMs. It is Linux compressed Loop image. |
Working with qemu-img: Examples
Below are some of the examples of working with qemu-img .
Create a new disk image with qemu-img create
Syntax:
qemu-img create -f fmt fname size Create raw disk image of size 10 GB:
$ qemu-img create -f raw ubuntu.img 10G
Formatting 'ubuntu.img', fmt=raw size=10737418240
$ qemu-img info ubuntu.img
image: ubuntu.img
file format: raw
virtual size: 10G (10737418240 bytes)
disk size: 0
Replace raw with qcow2, vdk, vmdk to create such formats. Have a look at below for vmdk:
$ qemu-img create -f vmdk ubuntu.vmdk 10G
Formatting 'ubuntu.vmdk', fmt=vmdk size=10737418240 compat6=off hwversion=undefined
$ qemu-img info ubuntu.vmdk
image: ubuntu.vmdk
file format: vmdk
virtual size: 10G (10737418240 bytes)
disk size: 12K
cluster_size: 65536
Format specific information:
cid: 1484281290
parent cid: 4294967295
create type: monolithicSparse
extents:
[0]:
virtual size: 10737418240
filename: ubuntu.vmdk
cluster size: 65536
format:
Convert disk image with qemu-img convert
Syntax:
qemu-img convert -O out_fmt fname out_fname Example: Convert vmdk image to qcow2
$ qemu-img create -f vmdk ~/ubuntu.vmdk 10G
Formatting '/home/josepy/ubuntu.vmdk', fmt=vmdk size=10737418240 compat6=off hwversion=undefined
$ qemu-img convert -O qcow2 ~/ubuntu.vmdk ~/ubuntu.qcow2
$ qemu-img info ~/ubuntu.qcow2
image: /home/josepy/ubuntu.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: falseCheck for disk with qemu-img check
Syntax:
qemu-img check -f fmt fname Example:
$ qemu-img check ubuntu.vmdk
No errors were found on the image.Resize disk image with qemu-img resize
WARNING: Before using this command to shrink a disk image, you MUST use file system and partitioning tools inside the VM to reduce allocated file systems and partition sizes accordingly. Failure to do so will result in data loss!
Syntax: resize filename [+ | -]size
Example: Increase disk image by 3GB
$ qemu-img create -f qcow2 ubuntu.qcow2 10G
Formatting 'ubuntu.qcow2', fmt=qcow2 size=10737418240 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ qemu-img info ubuntu.qcow2
image: ubuntu.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
$ qemu-img info ubuntu.qcow2
image: ubuntu.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
$ qemu-img resize ubuntu.qcow2 +3G
Image resized.
$ qemu-img info ubuntu.qcow2
image: ubuntu.qcow2
file format: qcow2
virtual size: 13G (13958643712 bytes)
disk size: 200K
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: falseYou should notice that the size was increased from 10GB to 13GB
NOTE: After enlarging the disk image, you must use file system and partitioning tools inside the virtual machine to actually begin using the new space.
- How to extend/increase KVM Virtual Machine (VM) disk size
- How to extend root filesystem using LVM in Linux
Managing snapshots with qemu-img snapshot
Snapshots are often used to save virtual machine in a particular state. It is a snapshot of the complete environment in which a VM Guest is running. A snapshot normally includes the state of:
- memory (RAM)
- the processor (CPU)
- devices
- And all writable disks.
In order to use snapshots, your VM Guest must contain at least one writable hard disk image in qcow2 format. This device is usually the first virtual hard disk. Working with qemu-img snapshots is illustrated below.
Example: Create of current state of VM with a name ubuntu_fresh
$ qemu-img snapshot -c ubuntu_fresh ubuntu.qcow2To list snapshots of VM , use:
$ qemu-img snapshot -l ubuntu.qcow2
Snapshot list:
ID TAG VM SIZE DATE VM CLOCK
1 ubuntu_fresh 0 2017-03-06 18:41:21 00:00:00.000If something breaks in your Virtual machine and you need to restore the state of the saved snapshot, you the id given by command above:
$ qemu-img snapshot -a 1 ubuntu.qcow2To delete snapshot:
$ qemu-img snapshot -d 1 ubuntu.qcow2Rebasing derived images
$ qemu-img rebase -b /new/ubuntu.raw ubuntu.qcow2Install OS to created disk image
To install an operating system into your disk image, you need the installation medium such as cdrom or ISO image file. The installation medium should not be mounted because QEMU accesses the media directly.
qemu-system-x86_64 \
-m 512\
-hda ~/Desktop/virt/arch.qcow2 \
-cdrom ~/iso/CentOS-7-x86_64-Everything-1611.iso \
-enable-kvmYou can specify multiple images: -hda ubuntu.qcow2 -hdb files.img -hdc container.qcow2. ISO image location can be cdrom device, -cdrom dev/cdrom
Further reading:
