QEMU - open source processor emulator

1. Download an ISO image
2. Set up a few variables :
   vmName='debian'; diskSize='10G'; ramSize='1G'; qemuWorkDir="$HOME/QEMU/"; mkdir -p "$qemuWorkDir"
3. Create a new disk image :
   qemu-img create -f qcow2 "$qemuWorkDir/$vmName.img" "$diskSize"
4. Boot an ISO image to install an operating system :
   qemu-system-x86_64 -enable-kvm -boot d -cdrom /path/to/image.iso -hda "$qemuWorkDir/$vmName.img" -m "$ramSize"
   When installing Debian :

   • proceed in Expert mode : this is more verbose if something goes wrong
   • at the Installing the base system stage, when offered 2 kernel versions to install :
     • don't pick the linux-image-amd64 version
     • select the linux-image-x.y.z version
5. Once installed, boot the system :
   qemu-system-x86_64 -enable-kvm -hda "$qemuWorkDir/$vmName.img" -m "$ramSize"

qemu-img convert -c myVirtualMachine.img -O qcow myVirtualMachine.img.compressed

• qcow is qemu's Copy On Write image format.
• myVirtualMachine.img.compressed is compressed and still runnable.

1. apt install bridge-utils
2. In /etc/network/interfaces, change the configuration method for the physical interface from auto to manual so that Network Manager (or equivalent) leaves it quiet :

   #auto eth0
   iface eth0 inet manual

3. Append to /etc/network/interfaces :

   # auto load a bridge
   auto br0
   
   # configure the bridge using DHCP
   iface br0 inet dhcp
   
   	# create a tap device owned by bob (who is a host system user), and turn it up
   	pre-up ip tuntap add dev tap0 mode tap user bob
   	pre-up ip link set tap0 up
   
   	# add all physical interfaces to the bridge
   	bridge_ports all tap0
   
   	# speed up the activating of the bridge (details)
   	bridge_stp		off
   	bridge_maxwait		0
   	bridge_fd		0
   
   	post-down ip link set tap0 down
   	post-down ip tuntap del dev tap0 mode tap

4. reload changes :
   • service networking restart
   • systemctl restart NetworkManager
5. Boot the virtual machine with the network management options (details) :
   qemu -hda imagefile.img -net nic -net tap,ifname=tap0,script=no,downscript=no
   The network adapter of the guest system will be configured by the guest system itself with /etc/network/interfaces and DHCP.
6. To run extra guests simultaneously, create some tap1, tap2, ..., tap devices by duplicating the lines containing tap0 in /etc/network/interfaces.
   • Don't forget to update the ifname=tap... argument of the command above when starting the guest systems.
   • If guests were made by cloning a "master" virtual machine, there are chances the MAC addresses are all the same. Read about how to fix this.

Create a snapshot :

Temporary snapshots

Update the base image

Commit changes made in a snapshot into the base image :

Enter the monitor (on a running machine)

CTRL-ALT-SHIFT-2

Leave the monitor

CTRL-ALT-SHIFT-1

Get info

info cpus

1. Define some shell variables :
   rootDir='/home/bob/'; isoImageFile="${rootDir}path/to/debian-7.3.0-amd64-netinst.iso"; vmDir="${rootDir}Qemu/"; vmFile="${vmDir}debian73x64.img"; vmDiskSizeGb=2; ramMb=1024; nbCpu=2
2. Create and install the new virtual machine :qemu-img create "$vmFile" "${vmDiskSizeGb}G"; qemu-system-x86_64 -smp $nbCpu -m $ramMb -boot d -hda "$vmFile" -cdrom "$isoImageFile"
3. Convert and compress the virtual machine image : compressedVmFile="${vmFile}.compressed"; qemu-img convert -c "$vmFile" -O qcow "$compressedVmFile"
4. Configure networking on the host system
5. Boot the virtual machine with network support to make sure it's ok :
   qemu-system-x86_64 -smp $nbCpu -m $ramMb -hda "$compressedVmFile" -net nic -net tap,ifname=tap0,script=no,downscript=no
   Once there :
   1. try some pings (to the virtual machine itself, the host system, any external host, ...)
   2. make sure you have all your CPUs : top 1
   3. while in top, check the RAM quantity
6. If several virtual machines were "snapshoted" from the same initial install :
   1. Assign different MAC addresses manually
   2. Assign static IP addresses manually
7. When ok, leave top with q and halt the virtual machine.
8. Start the virtual machine in temporary snapshot mode : qemu-system-x86_64 -smp $nbCpu -m $ramMb -hda "$compressedVmFile" -net nic -net tap,ifname=tap0,script=no,downscript=no -snapshot
9. Enjoy !