Use qemu-img create to create a new disk image for your VM Guest operating system. The command uses the following syntax:

> qemu-img create -f fmt1 -o options2 fname3 size4

To create a new disk image sles.raw in the directory /images growing up to a maximum size of 4 GB, run the following command:

> qemu-img create -f raw -o size=4G /images/sles.raw
Formatting '/images/sles.raw', fmt=raw size=4294967296

> ls -l /images/sles.raw
-rw-r--r-- 1 tux users 4294967296 Nov 15 15:56 /images/sles.raw

> qemu-img info /images/sles.raw
image: /images/sles11.raw
file format: raw
virtual size: 4.0G (4294967296 bytes)
disk size: 0

As you can see, the virtual size of the newly created image is 4 GB, but the actual reported disk size is 0 as no data has been written to the image yet.

Tip: VM Guest images on the Btrfs file system

If you need to create a disk image on the Btrfs file system, you can use nocow=on to reduce the performance overhead created by the copy-on-write feature of Btrfs:

> qemu-img create -o nocow=on test.img 8G

If you, however, want to use copy-on-write (for example for creating snapshots or sharing them across virtual machines), then leave the command line without the nocow option.

Use qemu-img convert to convert disk images to another format. To get a complete list of image formats supported by QEMU, run qemu-img -h and look at the last line of the output. The command uses the following syntax:

> qemu-img convert -c1 -f fmt2 -O out_fmt3 -o options4 fname5 out_fname6

> qemu-img convert -O vmdk /images/sles.raw \
/images/sles.vmdk

> ls -l /images/
-rw-r--r-- 1 tux users 4294967296 16. lis 10.50 sles.raw
-rw-r--r-- 1 tux users 2574450688 16. lis 14.18 sles.vmdk

To see a list of options relevant for the selected target image format, run the following command (replace vmdk with your image format):

> qemu-img convert -O vmdk /images/sles.raw \
/images/sles.vmdk -o ?
Supported options:
size             Virtual disk size
backing_file     File name of a base image
compat6          VMDK version 6 image
subformat        VMDK flat extent format, can be one of {monolithicSparse \
    (default) | monolithicFlat | twoGbMaxExtentSparse | twoGbMaxExtentFlat}
scsi             SCSI image

Use qemu-img check to check the existing disk image for errors. Not all disk image formats support this feature. The command uses the following syntax:

> qemu-img check -f fmt1 fname2

If no error is found, the command returns no output. Otherwise, the type and number of errors found is shown.

> qemu-img check -f qcow2 /images/sles.qcow2
ERROR: invalid cluster offset=0x2af0000
[...]
ERROR: invalid cluster offset=0x34ab0000
378 errors were found on the image.

When creating a new image, you must specify its maximum size before the image is created (see Section 33.2.2.1, “qemu-img create”). After you have installed the VM Guest and have been using it for certain time, the initial size of the image may no longer be sufficient. In that case, add more space to it.

To increase the size of an existing disk image by 2 gigabytes, use:

> qemu-img resize /images/sles.raw +2GB

Note

You can resize the disk image using the formats raw and qcow2. To resize an image in another format, convert it to a supported format with qemu-img convert first.

The image now contains an empty space of 2 GB after the final partition. You can resize the existing partitions or add new ones.

qcow2 is the main disk image format used by QEMU. Its size grows on demand, and the disk space is only allocated when it is needed by the virtual machine.

A qcow2 formatted file is organized in units of constant size. These units are called clusters. Viewed from the guest side, the virtual disk is also divided into clusters of the same size. QEMU defaults to 64 kB clusters, but you can specify a different value when creating a new image:

> qemu-img create -f qcow2 -o cluster_size=128K virt_disk.qcow2 4G

A qcow2 image contains a set of tables organized in two levels that are called the L1 and L2 tables. There is just one L1 table per disk image, while there can be many L2 tables depending on how big the image is.

To read or write data to the virtual disk, QEMU needs to read its corresponding L2 table to find out the relevant data location. Because reading the table for each I/O operation consumes system resources, QEMU keeps a cache of L2 tables in memory to speed up disk access.

disk_size = l2_cache_size * cluster_size / 8

disk_size = l2_cache_size * 8192

l2_cache_size = disk_size_GB * 131072

# qemu-system-ARCH [...] \
 -drive file=disk_image.qcow2,l2-cache-size=4194304,refcount-cache-size=262144

# qemu-system-ARCH [...] -drive file=hd.qcow2,cache-clean-interval=600

Use qemu-img snapshot -l DISK_IMAGE to view a list of all existing snapshots saved in the disk_image image. You can get the list even while the VM Guest is running.

> qemu-img snapshot -l /images/sles.qcow2
Snapshot list:
ID1       TAG2               VM SIZE3        DATE4          VM CLOCK5
1         booting                4.4M 2013-11-22 10:51:10   00:00:20.476
2         booted                 184M 2013-11-22 10:53:03   00:02:05.394
3         logged_in              273M 2013-11-22 11:00:25   00:04:34.843
4         ff_and_term_running    372M 2013-11-22 11:12:27   00:08:44.965

Use qemu-img snapshot -c SNAPSHOT_TITLE DISK_IMAGE to create a snapshot of the current state of a virtual machine that was previously powered off.

> qemu-img snapshot -c backup_snapshot /images/sles.qcow2

> qemu-img snapshot -l /images/sles.qcow2
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         booting                4.4M 2013-11-22 10:51:10   00:00:20.476
2         booted                 184M 2013-11-22 10:53:03   00:02:05.394
3         logged_in              273M 2013-11-22 11:00:25   00:04:34.843
4         ff_and_term_running    372M 2013-11-22 11:12:27   00:08:44.965
5         backup_snapshot           0 2013-11-22 14:14:00   00:00:00.000

If something breaks in your VM Guest and you need to restore the state of the saved snapshot (ID 5 in our example), power off your VM Guest and execute the following command:

> qemu-img snapshot -a 5 /images/sles.qcow2

The next time you run the virtual machine with qemu-system-ARCH, it will be in the state of snapshot number 5.

Note

The qemu-img snapshot -c command is not related to the savevm command of QEMU monitor (see Chapter 35, Virtual machine administration using QEMU monitor). For example, you cannot apply a snapshot with qemu-img snapshot -a on a snapshot created with savevm in QEMU's monitor.

Use qemu-img snapshot -d SNAPSHOT_ID DISK_IMAGE to delete old or unneeded snapshots of a virtual machine. This saves disk space inside the qcow2 disk image as the space occupied by the snapshot data is restored:

> qemu-img snapshot -d 2 /images/sles.qcow2

First, build a disk image as usual and install the target system on it. For more information, see Section 33.1, “Basic installation with qemu-system-ARCH” and Section 33.2.2, “Creating, converting, and checking disk images”. Then build a new image while using the first one as a base image. The base image is also called a backing file. After your new derived image is built, never boot the base image again, but boot the derived image instead. Several derived images may depend on one base image at the same time. Therefore, changing the base image can damage the dependencies. While using your derived image, QEMU writes changes to it and uses the base image only for reading.

It is a good practice to create a base image from a freshly installed (and, if needed, registered) operating system with no patches applied and no additional applications installed or removed. Later on, you can create another base image with the latest patches applied and based on the original base image.

Note

While you can use the raw format for base images, you cannot use it for derived images because the raw format does not support the backing_file option. Use for example the qcow2 format for the derived images.

For example, /images/sles_base.raw is the base image holding a freshly installed system.

> qemu-img info /images/sles_base.raw
image: /images/sles_base.raw
file format: raw
virtual size: 4.0G (4294967296 bytes)
disk size: 2.4G

The image's reserved size is 4 GB, the actual size is 2.4 GB, and its format is raw. Create an image derived from the /images/sles_base.raw base image with:

> qemu-img create -f qcow2 /images/sles_derived.qcow2 \
-o backing_file=/images/sles_base.raw
Formatting '/images/sles_derived.qcow2', fmt=qcow2 size=4294967296 \
backing_file='/images/sles_base.raw' encryption=off cluster_size=0

Look at the derived image details:

> qemu-img info /images/sles_derived.qcow2
image: /images/sles_derived.qcow2
file format: qcow2
virtual size: 4.0G (4294967296 bytes)
disk size: 140K
cluster_size: 65536
backing file: /images/sles_base.raw \
(actual path: /images/sles_base.raw)

Although the reserved size of the derived image is the same as the size of the base image (4 GB), the actual size is 140 KB only. The reason is that only changes made to the system inside the derived image are saved. Run the derived virtual machine, register it, if needed, and apply the latest patches. Do any other changes in the system such as removing unneeded or installing new software packages. Then shut the VM Guest down and examine its details once more:

> qemu-img info /images/sles_derived.qcow2
image: /images/sles_derived.qcow2
file format: qcow2
virtual size: 4.0G (4294967296 bytes)
disk size: 1.1G
cluster_size: 65536
backing file: /images/sles_base.raw \
(actual path: /images/sles_base.raw)

The disk size value has grown to 1.1 GB, which is the disk space occupied by the changes on the file system compared to the base image.

After you have modified the derived image (applied patches, installed specific applications, changed environment settings, etc.), it reaches the desired state. At that point, you can merge the original base image and the derived image to create a new base image.

Your original base image (/images/sles_base.raw) holds a freshly installed system. It can be a template for new modified base images, while the new one can contain the same system as the first one plus all security and update patches applied, for example. After you have created this new base image, you can use it as a template for more specialized derived images as well. The new base image becomes independent of the original one. The process of creating base images from derived ones is called rebasing:

> qemu-img convert /images/sles_derived.qcow2 \
-O raw /images/sles_base2.raw

This command created the new base image /images/sles_base2.raw using the raw format.

> qemu-img info /images/sles_base2.raw
image: /images/sles11_base2.raw
file format: raw
virtual size: 4.0G (4294967296 bytes)
disk size: 2.8G

The new image is 0.4 gigabytes bigger than the original base image. It uses no backing file, and you can easily create new derived images based upon it. This lets you create a sophisticated hierarchy of virtual disk images for your organization, saving a lot of time and work.

It can be useful to mount a virtual disk image under the host system. It is strongly recommended to read Chapter 18, libguestfs and use dedicated tools to access a virtual machine image. However, if you need to do this manually, follow this guide.

Linux systems can mount an internal partition of a raw disk image using a loopback device. The first example procedure is more complex but more illustrative, while the second one is straightforward:

Warning: Do not write to images currently in use

Never mount a partition of an image of a running virtual machine in a read-write mode. This could corrupt the partition and break the whole VM Guest.