QEMU/Images
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Once
QEMU has been installed, and kqemu compiled and configured, it should
be ready to run a guest OS from a virtual disc image. A disc image is a
file that represents the data on a hard disc. From the perspective of
the guest OS, it actually is a hard disc, and the guest OS can actually
create its own filesystem on the virtual disc.
You can download a few guest OS images from the QEMU website, including a simple 8MB image of a Linux distro. To run it, download and unzip the image in a folder and run the QEMU command.
qemu linux-0.2.img
Replace linux-0.2.img
with the name of your guest OS image file. If it has a GUI and you want
to use your mouse with it, double-click on the window and QEMU will
grab your mouse. To make QEMU release your mouse again, hold down the
Control and Alt keys simultaneously, then let go – your mouse will be
released back to X.
Image types
QEMU supports several image types. The “native” and most flexible type is qcow2, which supports copy on write, encryption, compression, and VM snapshots.
If you need to copy files to and from the image directly from the host, however, you need to use the raw image type.
QEMU currently supports these image types or formats:
raw
Creating an image
To set up your own guest OS image, you first need to create a blank disc image. QEMU has the qemu-img
command for creating and manipulating disc images, and supports a
variety of formats. If you don’t tell it what format to use, it will use
raw files. The “native” format for QEMU is qcow2, and this format
offers some flexibility. Here we’ll create a 3GB qcow2 image to install
Windows XP on:
qemu-img create -f qcow2 winxp.img 3GB
The
easiest way to install a guest OS is to create an ISO image of a boot
CD/DVD and tell QEMU to boot off it. Many free operating systems can be
downloaded from the Internet as bootable ISO images, and you can use
them directly without having to burn them to disc.
Here
we’ll boot off an ISO image of a properly licensed Windows XP boot
disc. We’ll also give it 256MB of RAM, but we won’t use the kqemu kernel
module just yet because it causes problems during Windows XP
installation.
qemu -m 256 -hda winxp.img -cdrom winxpsp2.iso -boot d
To boot from a real CD or DVD, tell QEMU where to find it. On Linux systems, you can usually use a logical device name like /dev/cdrom or /dev/dvd, or the physical name of the device, e.g. /dev/sr0
qemu -m 256 -hda winxp.img -cdrom /dev/cdrom -boot d
QEMU
will boot from the ISO image or CD/DVD and run the install program. If
you have two screens, move the QEMU screen off to the spare one where
you can keep an eye on the installer, but get on with something else –
it will take a while!
Once
the guest OS has installed successfully, you can shutdown the guest OS
(e.g. in Windows XP, click on Start and then Shutdown). Once it has
shutdown, start QEMU up with the kqemu kernel module to give it a little
more speed.
qemu -m 256 -hda winxp.img -cdrom winxpsp2.iso -kernel-kqemu
If you are running an x86-64 Linux (i.e. 64-bit), you will need to run the x86-64 version of QEMU to be able to utilise kqemu:
qemu-system-x86_64 -m 256 -hda winxp.img -cdrom winxpsp2.iso -kernel-kqemu
Using multiple images
QEMU
can utilise up to four image files to present multiple virtual drives
to the guest system. This can be quite useful, as in the following
examples:
a pagefile or swapfile virtual disc that can be shared between QEMU guests
Bear in mind that only one instance of QEMU may access an image at a time – shared doesn’t mean shared simultaneously!
To utilise additional images in QEMU, specify them on the command line with options -hda, -hdb, -hdc, -hdd.
qemu -m 256 -hda winxp.img -hdb pagefile.img -hdc testdata.img -hdd tempfiles.img -kernel-kqemu
NB:
QEMU doesn’t support both -hdc and -cdrom at the same time, as they
both represent the first device on the second IDE channel.
Copy on write
The “cow” part of qcow2 is an acronym for copy on write,
a neat little trick that allows you to set up an image once and use it
many times without changing it. This is ideal for developing and testing
software, which generally requires a known stable environment to start
off with. You can create your known stable environment in one image, and
then create several disposable copy-on-write images to work in.
To start a new disposable environment based on a known good image, invoke the qemu-img
command with the option -b and tell it what image to base its copy on.
When you run QEMU using the disposable environment, all writes to the
virtual disc will go to this disposable image, not the base copy.
qemu-img create -f qcow2 -b winxp.img test01.img 3GB
qemu -m 256 -hda test01.img -kernel-kqemu &
NB:
don’t forget to copy any important data out of the disposable
environment before deleting it. When developing and testing software in
copy-on-write virtual environments, it is a good idea to use version control software like Subversion or CVS
on a server external to your virtual environment. Not only is it easy
to keep copies of your work outside your virtual environment, it is also
very easy to set up a new virtual environment from version control.
Mounting an image on the host
Sometimes
it is helpful to be able to mount a drive image under the host system.
For example, if the guest doesn’t have network support, the only way to
transfer files into and out of the guest will be by the storage devices
it can address.
Linux and other Unix-like hosts can mount images created with the raw format type using a loopback device. From a root login (or using sudo), mount a loopback with an offset of 32,256.
mount -o loop,offset=32256 /path/to/image.img /mnt/mountpoint
For example, to copy some files across to a FreeDOS hard drive image:
mkdir -p /mnt/freedos
mount -o loop,offset=32256 freedos-c.img /mnt/freedos
cp oldgames /mnt/freedos
umount /mnt/freedos
NB: never mount a QEMU image while QEMU is using it, or you are likely to corrupt the filesystem on the image.
Note:
if you have an image without partitions you should omit the
,offset=32256 part. This is for instance the case if you want to mount
linux-0.2.img (which can be found at the qemu web site at the time of
writing)
Getting information
The qemu-img program can tell you about the format, virtual size, physical size, and snapshots inside an image.
$ qemu-img info test.vmdk
(VMDK) image open: flags=0x2 filename=test.vmdk
image: test.vmdk
file format: vmdk
virtual size: 20M (20971520 bytes)
disk size: 17M
Converting image formats
The qemu-img
program can be used to convert images from one format to another, or
add compression or encryption to an image. Specify the source and target
files for the image, and select from the following options:
-f – optional, specify the format of the input file (QEMU can usually detect it)qemu-img convert -O qcow2 test.vmdk test.qcow2
Exchanging images with VirtualBox
To convert a QEMU image for use with VirtualBox, first convert it to raw
format, then use VirtualBox’s conversion utility to convert and compact
it in its native format. Note that the compact command requires the
full path to the VirtualBox image, not just the filename.
qemu-img convert -O raw test.qcow2 test.raw
(1) VBoxManage convertdd test.raw test.vdi
VBoxManage modifyvdi /full/path/to/test.vdi compact
(1) or try :
VBoxManage convertfromraw -format VDI test.raw test.vdi
VBoxManage
doesn’t have the capability to convert an image back to raw format.
However, an older tool was available for download off the VirtualBox
website, vditool, can apparently convert from .vdi to raw format. UPDATE – Feb 2009 – current installations of VirtualBox should already include vditool. Kvditool is a GUI for managing vditool.
(2) or try :
VBoxManage internalcommands converttoraw file.vdi file.raw
