Module 12 – Building a VM with KVM

• Introduction
• Disk Creation
• Disk Conversion
• Disk Shrinking
• VM Creation
• Installing

Introduction

In this guide we are going to build a VM using KVM

Disk Creation

KVM typically stores virtual machine hdd images in single qcow2 format file and minimega provides a way to create those files.

Note: A 100G file is not created. This is just the maximum size the VM can grow to.

minimega:/tmp/minimega/minimega$ disk create qcow2 /home/ubuntu/test.qcow2 100G

You can create disks in more formats using the qemu-img command.

root@ubuntu:~/minimega# qemu-img create -f qcow /home/ubuntu/test.img 1G
Formatting '/home/ubuntu/test.img', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16

You can also enable things like compression if you want to sacrifice speed for file size.

qemu-img convert -f qcow -O qcow -c /home/ubuntu/test.img /home/ubuntu/test-compressed.qcow

Disk Conversion

Sometimes you will need to convert disk images from one file type to another. Such as qcow->qcow2, vmdk->qcow2, or qcow2->qcow. Thankfully qemu-img convert can be used for this.

root@ubuntu:~/minimega# qemu-img create -f qcow /home/ubuntu/file.qcow 1G
Formatting '/home/ubuntu/file.qcow', fmt=qcow size=1073741824 encryption=off
root@ubuntu:~/minimega# qemu-img convert -f qcow -O qcow2 /home/ubuntu/file.qcow /home/ubuntu/file.qcow2

Disk Shrinking

There is a side effect to the way virtual image formats work. If you create a large file say 50GB and then delete the large file.

The file on disk will have grown to 50GB+ and will not shrink back down on its own. Shrinking a disk is a very time consuming process.

You need to create a file of all zeros that fills the disk and then delete it.

With a Windows Guest this can be done with fsutil

fsutil file createnew myfile 100000000000
del myfile

With a Linux Guest you can use dd

dd if=/dev/zero of=/mytempfile
rm -f /mytempfile

When that is complete you can convert the disk with qemu-img convert

mv image.qcow2 image.qcow2_backup
qemu-img convert -O qcow2 image.qcow2_backup image.qcow2

VM Creation

Now we have a disk image, we can use this in minimega to save files. Notice how we turn snapshot to false. When snapshot is false all writes are committed to a disk. When you kill and remove a VM the changes will remain.

Lets use the ISO module 0 to install Ubuntu Server to a disk image.

minimega:/tmp/minimega/minimega$ disk create qcow2 /home/ubuntu/ubuntuserver.qcow2 100G
minimega:/tmp/minimega/minimega$ clear vm config
minimega:/tmp/minimega/minimega$ vm config disk /home/ubuntu/ubuntuserver.qcow2
minimega:/tmp/minimega/minimega$ vm config cdrom /home/ubuntu/ubuntuserver.iso
minimega:/tmp/minimega/minimega$ vm config memory 1024
minimega:/tmp/minimega/minimega$ vm config snapshot false
minimega:/tmp/minimega/minimega$ vm launch kvm ubuntuserver
minimega:/tmp/minimega/minimega$ vm start ubuntuserver

Installing

Connect to the VNC session and finish the install following the guide in Module 1.

Authors

The minimega authors

30 May 2017