36 Guest installation #
The libvirt
-based tools such as
virt-manager
and virt-install
offer
convenient interfaces to set up and manage virtual machines. They act as a
kind of wrapper for the qemu-system-ARCH
command.
However, it is also possible to use qemu-system-ARCH
directly without using
libvirt
-based tools.
qemu-system-ARCH
and libvirt
Virtual Machines created with
qemu-system-ARCH
are not visible for the
libvirt
-based tools.
36.1 Basic installation with qemu-system-ARCH
#
In the following example, a virtual machine for a SUSE Linux Enterprise Server 11 installation is created. For detailed information on the commands, refer to the respective man pages.
If you do not already have an image of a system that you want to run in a virtualized environment, you need to create one from the installation media. In such case, you need to prepare a hard disk image, and obtain an image of the installation media or the media itself.
Create a hard disk with qemu-img
.
>
qemu-img create1 -f raw2 /images/sles/hda3 8G4
The subcommand | |
Specify the disk's format with the | |
The full path to the image file. | |
The size of the image, 8 GB in this case. The image is created as a Sparse image file that grows when the disk is filled with data. The specified size defines the maximum size to which the image file can grow. |
After at least one hard disk image is created, you can set up a virtual
machine with qemu-system-ARCH
that boots into the
installation system:
#
qemu-system-x86_64 -name "sles"1-machine accel=kvm -M pc2 -m 7683 \
-smp 24 -boot d5 \
-drive file=/images/sles/hda,if=virtio,index=0,media=disk,format=raw6 \
-drive file=/isos/SLE-15-SP6-Online-ARCH-GM-media1.iso,index=1,media=cdrom7 \
-net nic,model=virtio,macaddr=52:54:00:05:11:118 -net user \
-vga cirrus9 -balloon virtio10
Name of the virtual machine that is displayed in the window caption and be used for the VNC server. This name must be unique. | |
Specifies the machine type. Use | |
Maximum amount of memory for the virtual machine. | |
Defines an SMP system with two processors. | |
Specifies the boot order. Valid values are | |
Defines the first ( | |
The second ( | |
Defines a paravirtualized ( | |
Specifies the graphic card. If you specify | |
Defines the paravirtualized balloon device that allows to dynamically
change the amount of memory (up to the maximum value specified with
the parameter |
After the installation of the guest operating system finishes, you can start the related virtual machine without the need to specify the CD-ROM device:
#
qemu-system-x86_64 -name "sles" -machine type=pc,accel=kvm -m 768 \
-smp 2 -boot c \
-drive file=/images/sles/hda,if=virtio,index=0,media=disk,format=raw \
-net nic,model=virtio,macaddr=52:54:00:05:11:11 \
-vga cirrus -balloon virtio
36.2 Managing disk images with qemu-img
#
In the previous section (see
Section 36.1, “Basic installation with qemu-system-ARCH
”), we used the
qemu-img
command to create an image of a hard disk.
You can, however, use qemu-img
for general disk image
manipulation. This section introduces qemu-img
subcommands to help manage the disk images flexibly.
36.2.1 General information on qemu-img invocation #
qemu-img
uses subcommands (like
zypper
does) to do specific tasks. Each subcommand
understands a different set of options. Certain options are general and
used by more of these subcommands, while others are unique to the
related subcommand. See the qemu-img man page (man 1
qemu-img
) for a list of all supported options.
qemu-img
uses the following general syntax:
>
qemu-img subcommand [options]
and supports the following subcommands:
create
Creates a new disk image on the file system.
check
Checks an existing disk image for errors.
compare
Check if two images have the same content.
map
Dumps the metadata of the image file name and its backing file chain.
amend
Amends the image format specific options for the image file name.
convert
Converts an existing disk image to a new one in a different format.
info
Displays information about the relevant disk image.
snapshot
Manages snapshots of existing disk images.
commit
Applies changes made to an existing disk image.
rebase
Creates a new base image based on an existing image.
resize
Increases or decreases the size of an existing image.
36.2.2 Creating, converting, and checking disk images #
This section describes how to create disk images, check their condition, convert a disk image from one format to another, and get detailed information about a particular disk image.
36.2.2.1 qemu-img create #
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
The format of the target image. Supported formats are
| |
Certain image formats support additional options to be passed on the
command line. You can specify them here with the
| |
Path to the target disk image to be created. | |
Size of the target disk image (if not already specified with the
|
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.
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.
36.2.2.2 qemu-img convert #
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
Applies compression on the target disk image. Only
| |
The format of the source disk image. It is normally autodetected and can therefore be omitted. | |
The format of the target disk image. | |
Specify additional options relevant for the target image format.
Use | |
Path to the source disk image to be converted. | |
Path to the converted target disk image. |
>
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
36.2.2.3 qemu-img check #
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
The format of the source disk image. It is normally autodetected and can therefore be omitted. | |
Path to the source disk image to be checked. |
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.
36.2.2.4 Increasing the size of an existing disk image #
When creating a new image, you must specify its maximum size before the image is created (see Section 36.2.2.1, “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
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.
36.2.2.5 Advanced options for the qcow2 file format #
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.
36.2.2.5.1 Choosing the right cache size #
The cache size relates to the amount of allocated space. L2 cache can map the following amount of virtual disk:
disk_size = l2_cache_size * cluster_size / 8
With the default 64 kB of cluster size, that is
disk_size = l2_cache_size * 8192
Therefore, to have a cache that maps n
gigabytes
of disk space with the default cluster size, you need
l2_cache_size = disk_size_GB * 131072
QEMU uses 1 MB (1048576 bytes) of L2 cache by default. Following the above formulas, 1 MB of L2 cache covers 8 GB (1048576 / 131072) of virtual disk. This means that the performance is fine with the default L2 cache size if your virtual disk size is up to 8 GB. For larger disks, you can speed up the disk access by increasing the L2 cache size.
36.2.2.5.2 Configuring the cache size #
You can use the -drive
option on the QEMU
command line to specify the cache sizes. Alternatively when
communicating via QMP, use the blockdev-add
command. For more information on QMP, see
Section 38.11, “QMP - QEMU machine protocol”.
The following options configure the cache size for the virtual guest:
- l2-cache-size
The maximum size of the L2 table cache.
- refcount-cache-size
The maximum size of the refcount block cache. For more information on refcount, see https://raw.githubusercontent.com/qemu/qemu/master/docs/qcow2-cache.txt.
- cache-size
The maximum size of both caches combined.
When specifying values for the options above, be aware of the following:
The size of both the L2 and refcount block caches needs to be a multiple of the cluster size.
If you only set one of the options, QEMU automatically adjusts the other options so that the L2 cache is 4 times bigger than the refcount cache.
The refcount cache is used much less often than the L2 cache, therefore you can keep it small:
#
qemu-system-ARCH [...] \
-drive file=disk_image.qcow2,l2-cache-size=4194304,refcount-cache-size=262144
36.2.2.5.3 Reducing the memory usage #
The larger the cache, the more memory it consumes. There is a separate L2 cache for each qcow2 file. When using a lot of big disk images, you may need a considerably large amount of memory. Memory consumption is even worse if you add backing files (Section 36.2.4, “Manipulate disk images effectively”) and snapshots (see Section 36.2.3, “Managing snapshots of virtual machines with qemu-img”) to the guest's setup chain.
This is why QEMU introduced the
cache-clean-interval
setting. It defines an
interval in seconds after which all cache entries that have not
been accessed are removed from memory.
The following example removes all unused cache entries every 10 minutes:
#
qemu-system-ARCH [...] -drive file=hd.qcow2,cache-clean-interval=600
If this option is not set, the default value is 0 and it disables this feature.
36.2.3 Managing snapshots of virtual machines with qemu-img #
Virtual Machine snapshots are snapshots of the complete environment in which a VM Guest is running. The snapshot includes the state of the processor (CPU), memory (RAM), devices, and all writable disks.
Snapshots are helpful when you need to save your virtual machine in a particular state. For example, after you configured network services on a virtualized server and want to quickly start the virtual machine in the same state you last saved it. Or you can create a snapshot after the virtual machine has been powered off to create a backup state before you try something experimental and make VM Guest unstable. This section introduces the latter case, while the former is described in Chapter 38, Virtual machine administration using QEMU monitor.
To use snapshots, your VM Guest must contain at least one writable
hard disk image in qcow2
format. This device is
normally the first virtual hard disk.
Virtual Machine snapshots are created with the
savevm
command in the interactive QEMU monitor. To
make identifying a particular snapshot easier, you can assign it a
tag. For more information on QEMU monitor, see
Chapter 38, Virtual machine administration using QEMU monitor.
Once your qcow2
disk image contains saved snapshots,
you can inspect them with the qemu-img snapshot
command.
Do not create or delete virtual machine snapshots with the
qemu-img snapshot
command while the virtual
machine is running. Otherwise, you may damage the disk image with the
state of the virtual machine saved.
36.2.3.1 Listing existing snapshots #
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
Unique auto-incremented identification number of the snapshot. | |
Unique description string of the snapshot. It is meant as a human-readable version of the ID. | |
The disk space occupied by the snapshot. The more memory is consumed by running applications, the bigger the snapshot is. | |
Time and date the snapshot was created. | |
The current state of the virtual machine's clock. |
36.2.3.2 Creating snapshots of a powered-off virtual machine #
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.
The qemu-img snapshot -c
command is not related
to the savevm
command of QEMU monitor (see
Chapter 38, 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.
36.2.3.3 Deleting snapshots #
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
36.2.4 Manipulate disk images effectively #
Imagine the following real-life situation: you are a server administrator who runs and manages several virtualized operating systems. One group of these systems is based on one specific distribution, while another group (or groups) is based on different versions of the distribution or even on a different (and maybe non-Unix) platform. To make the case even more complex, individual virtual guest systems based on the same distribution differ according to the department and deployment. A file server typically uses a different setup and services than a Web server does, while both may still be based on SUSE® Linux Enterprise Server.
With QEMU it is possible to create “base” disk images. You can use them as template virtual machines. These base images save you plenty of time because you do not need to install the same operating system more than once.
36.2.4.1 Base and derived images #
First, build a disk image as usual and install the target system on
it. For more information, see
Section 36.1, “Basic installation with qemu-system-ARCH
” and
Section 36.2.2, “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.
36.2.4.2 Creating derived images #
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.
36.2.4.3 Rebasing derived images #
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.
36.2.4.4 Mounting an image on a VM Host Server #
It can be useful to mount a virtual disk image under the host system. It is strongly recommended to read Chapter 21, 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:
Set a loop device on the disk image whose partition you want to mount.
>
losetup /dev/loop0 /images/sles_base.rawFind the sector size and the starting sector number of the partition you want to mount.
>
fdisk -lu /dev/loop0 Disk /dev/loop0: 4294 MB, 4294967296 bytes 255 heads, 63 sectors/track, 522 cylinders, total 8388608 sectors Units = sectors of 1 * 512 = 5121 bytes Disk identifier: 0x000ceca8 Device Boot Start End Blocks Id System /dev/loop0p1 63 1542239 771088+ 82 Linux swap /dev/loop0p2 * 15422402 8385929 3421845 83 LinuxCalculate the partition start offset:
sector_size * sector_start = 512 * 1542240 = 789626880
Delete the loop and mount the partition inside the disk image with the calculated offset on a prepared directory.
>
losetup -d /dev/loop0>
mount -o loop,offset=789626880 \ /images/sles_base.raw /mnt/sles/>
ls -l /mnt/sles/ total 112 drwxr-xr-x 2 root root 4096 Nov 16 10:02 bin drwxr-xr-x 3 root root 4096 Nov 16 10:27 boot drwxr-xr-x 5 root root 4096 Nov 16 09:11 dev [...] drwxrwxrwt 14 root root 4096 Nov 24 09:50 tmp drwxr-xr-x 12 root root 4096 Nov 16 09:16 usr drwxr-xr-x 15 root root 4096 Nov 16 09:22 varCopy one or more files onto the mounted partition and unmount it when finished.
>
cp /etc/X11/xorg.conf /mnt/sles/root/tmp>
ls -l /mnt/sles/root/tmp>
umount /mnt/sles/
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.