Linux Virtualization with KVM
You are free to use this I use this blog like a wiki to write things I learn.
Here are some other Open Source Virtualization projects.
KVM - Kernel-based Virtual Machine
Xen
VirtualBox
UML - User Mode Linux
LXD/LXC
Docker
What is KVM?
KVM is a hypervisor built into the Linux Kernel and it
allows Linux desktop or server to simulate multiple piece of hardware.
KVM uses the QEMU virtual machine format so what are the difference between QEMU and KVM. First, they are two separate software projects and QEMU is primary a
hardware emulator. KVM is a kernel module that is used to expose hardware
virtualization technologies such as:
Intel VT-x or AMD SVM
KVM then uses QEMU for the device emulation.
When QEMU and KVM work together, KVM arbitrates access to the CPU
and memory, while QEMU emulates hardware resources like hard disk, video card,
USBs and more.
Hardware Virtualization
Full virtualization
Complete simulation of the actual hardware to allow software, which typically consists of the guest machine or virtual machine.
KVM uses full virtualization.
Para virtualization
The hardware environment is not simulated.
XEN Supports this not KVM.
Types of Hypervisors
VMM/hypervisor is a piece of software that is responsible for monitoring
and controlling virtual machines or guest operations systems
Type 1 hypervisor
- The hypervisor runs directly on top of the hardware.
Type 2 hypervisor
- The hypervisor acts as a separate layer often on top
of a base operating system.
Libvirt
Libvirt is a set of API libraries that sites in between the end user and the
hypervisor
The hypervisor could be built to use any virtualization technology such
as KVM/QEMU, XEN, LXC, VirtualBox, VMWARE ESX, MS HyperV and
even Parallels
Libvirt acts as a transparent layer that can take commands from
users modify them based on the underlying virtualization technology,
and then executes them on the actual hypervisor
Tools include the libvirtd daemon, API library, and command line utility
called virsh
Linux virtualization tips
- Overcommitting
To allocate more virtualized CPUs or virtual memory than the available
resources on the host system provides. Overcommitting can cause risk to your host system’s stability.
- Thin provisioning
Allows you to optimize the available storage space the guest virtual machines.
Like overcommitting, but only pertains to storage, not CPU and memory and
can also risk the system stability
Linux Virtualization are also used in cloud solutions.
OpenStack
Eucalytus
Cloudstack
Installing Virtualization Packages
To check for KVM support lsmod | grep kvm
Then check for this for AMD cpu the kvm_amd and for for INTEL cpu kvm_intel so if these flags show up then we have cpu support for KVM.
First, we install these packages, so we can use Linux virtualization.
The package libguestfs-tool is for virt-builder and is not needed for kvm
but is usefull and virt-manager is a gui manager for Linux virtualization and
you can skip it for servers.
Centos7
yum install -y qemu-kvm libvirt virt-install libguestfs-tool-c virt-manager
Fedora
dnf install -y qemu-kvm libvirt virt-install libguestfs-tool-c virt-manager
Groupinstall for Fedora
dnf install -y @virtualization
Ubuntu/debian systems
apt install -y qemu-kvm libvirt virt-install libguestfs-tools virt-manager
To list your machine capabilities virsh domcapabilities
libvirt-based-tools to manage virtual machines
First tool to manage virtual machines is the virsh shell
Hers is some commands.
To list all machines running
virsh list --all
This only show running virt-machines.
virsh list
To start a virt-machine
virsh start machine
It`s important to remember and check on the virtual machine what the name of the disk is called before attaching the disk-image.
To attach a disk to a virt-machine
virsh attach-disk nameofmachine --source /path/to/disk.format --tar get namelike-vdc --persistent
To stopp a virt-machine.
virsh shutdown yourmachine
to start a virt-machine.
virsh start
to reboot a virt-machine.
virsh reboot
to pause a virt-machine.
virsh suspend
Since virsh is a command line tool then use
virt-viewer to see the console or gui inside the
virtual machine like this.
virt-viewer yourmachine
Then the last tool to manage virtual machines is the gui tool
virt-manager and it can start stop take snapshots
add disk-images manage storage pools and create and delete network bridges.
Virt-manager can also manage other machines hypervisor and virtual machines
with ssh.
Tools for managing disk-images and install virtual machine
virt-builder creates ready to use virt-builder creates disk images for the most
common Linux distros so you not need to install them with a iso.
Here is a virt-builder command for more options see the manpage.
virt-builder ubuntu-18.04 --root-password password:yourpassword --size 51G --format qcow2
When managing disk-images you use the utility qemu-img to create new empty images like this and see the manpage for more options.
qemu-img create -f yourformat diskname.format 1G
To setup a virtual machine to use the disk-image you can use virt-manager or
use the virt-install command line utility like this.
virt-install --name distro --ram 1024 --vcpu=1 --network bridge=virbr0 --disk path=/path/to/disk-image.qcow2 --import
Or you can have more options like when install ing with a iso.
virt-install --name yourmachine --ram 1100 --disk path=/path/to/disk.format --vcpu 1 --os-type=youros --network bridge=virbr0 --graphics vnc,port=portnumber --console pty,target_type=serial --cdrom /path/to/iso
The tool qemu-img can also convert a disk-image from raw to qcow2 like this.
qemu-img convert -f raw -O qcow2 yourimage.img yourimage.qcow2
snapshots
With libvirt there is internal snapshot and external snapshot
Internal snapshot is everything inside a qcow2 fileformat.
Internal snapshot does pause the machine, so it’s not recommended to use on important production machines when uptime is important and internal snapshot is only for qcow2 disk-format.
It’s better to use them when you are having maintenance on a production
machine.
External snapshot is based on the concept copy on write file-system. Then a
exsternal snapshot is read only.
Exsternal snapshots do grow in tandem with the virtual-machines and it support all the disk-images formats in libivrt.
To list snapshots
virsh snapshot-list yourmachine.
Internal snapshot does this command.
virsh snapshot-create yourmachine
exsternal snapshot like this.
virsh snapshot-create-as diskname snap1-diskname
“snap1-diskname-description”
–diskspec hda,file=/export/vmimgs/snap2-disknameqcow2
–disk-only –atomic
To revert to a snapshot, use this command.
virsh snapshort-revert yourmachine --snapshotname "nameofsnap"
Bridge Networking
Virtual machines need networking to work so then we use bridge networking to
allow virtual interfaces to connect to the outside network through the physical
interface, making them appear as normal host to the rest for the network.
When installing Linux virtualization packages, it will setup a standard bridge
network for you called virbr0 and it’s using NAT to allow the virtual machine
to connect to the outside network.
There are many tools to manage Bridge networking like.
Brctl
Virt-manager -> Edit -> Connection-Details -> Virtual-Networks
Systemd-networkd
NetworkManager
link to archlinux-wiki site to learn more.