Virtualization

The typical virtualization stack on Linux is referred to as QEMU/KVM; both of these are separate technologies.

Tasks

Check CPU for virtualization support

grep -E 'svm|vmx' /proc/cpuinfo # (1)

1. AMD CPUs will have svm in the flags section, whereas Intel CPUs will have vmx.

Virtual machine

The easiest way to create a VM is with the Boxes GNOME Desktop Environment application or virt-manager.

virt-install \
    --cdrom=/tmp/debian-9.0.0-amd64-netinst.iso \
    --vcpus=1 --memory=1024 --disk size=5 \
    --os-variant=debian8
    --name=linuxconfig-vm \

Virtual networking

Using Boxes, a new interface will be created (apparently named tap0, etc) and slaved to a bridge, or a bridge will be created if none exist.

Console access

In order to enable console access to a domain, the serial-gettty@.service service must be started.

systemctl enable --now serial-getty@ttyS0.service

Now console access is available from the host

virsh console $DOMAIN

Note that the console will remain at 24 lines by 80 columns no matter if the terminal window is resized.

Virtual disk

CoW filesystems

If the disk image is created on a filesystem that does not support O_DIRECT (ref. man 5 open), i.e. COW filesystem like btrfs and ZFS, the cache must be disabled. This appears to be impossible on btrfs, so disk images must be created on partitions with non-CoW alternatives, like ext4 and xfs.

A virtual disk can be created in various ways and in various formats (the sparse QCOW2 disk image format, associated with QEMU, is preferred).

One way is to create the image using a utility like qemu-img.

Create qcow2 disk image

qemu-img create -f qcow2 disk0.qcow2 5G # (1)

1. By omitting -f, qemu-img will create a RAW format file.

Alternatively, and more circuitously, a volume can be created within a storage pool.

virsh pool-define-as --name Disks --type dir --target /disk/Disks # (1)
virsh pool-start Disks # (2)
virsh vol-create-as Disks disk0.qcow2 10G --format qcow2

1. A pool is deleted by first making sure its contents are deleted (but not the containing folder, in the case of a directory-based storage pool)

   virsh pool-destroy Disks  # Destroy the contents
   virsh pool-delete Disks   # Delete directory
   virsh pool-undefine Disks # Delete resource
   

2. Alternatively, activate pool on boot

   virsh pool-autostart Disks
   

Regardless of the method, the disk image is then attached to the domain, whether or not it is running.

virsh attach-disk rhel \
    --source /tmp/Disks/disk0.qcow2 \
    --target vdb --cache none \
    --driver qemu --subdriver qcow2 \
    --persistent # (1)

1. --persistent is necessary for the disk to remain attached after a shutdown. If the domain is not running, --config is necessary.

   virsh attach-disk rhel \
       --source /tmp/Disks/disk0.qcow2 \
       --target vdb --cache none \
       --driver qemu --subdriver qcow2 \
       --config
   

   This can be reversed with the following command.

   virsh detach-disk rhel /tmp/Disks/disk0.qcow2
   virhs detach-disk rhel vdb # Specifying target instead
   

Snapshots

Snapshots of VMs can be taken from within Boxes or via the command-line:

virsh snapshot-create-as rhel --name "Disks added"

These produce XML records that can then be viewed: note that the field that Boxes uses to identify each snapshot is actually "description".

virsh snapshot-dumpxml rhel "Disks added"

Snapshots can be renamed by changing the value of the name element in a text editor.

virsh snapshot-edit rhel --snapshotname "Disks added" --rename

Revert to a snapshot

virsh snapshot-revert rhel --snapshotname 

Custom resolution

Specify a custom resolution in a VM using KVM

cvt 2560 1440
xrandr --newmode "2560x1440_60.00" 312.25 2560 2752 3024 3488  1440 1443 1448 1493 -hsync +vsync
xrandr --addmode Virtual-1 2560x1440_60.00
xrandr --output Virtual-1 --mode 2560x1440_60.0

On a Hyper-V VM, this method will not work, but a simple change to a line in /etc/default/grub will do the trick after running update-grub and restarting

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=hyperv_fb:1920x1080"

Commands

virt-install

virsh

virsh is the main interface for managing libvirt guest domains. In virsh terminology, the word domain refers to a VM.

virsh commands can be entered from the virsh shell or from the command-line.

virsh list --all
virsh start rhel
virsh shutdown rhel
virsh destroy rhel  # Forcefully
virsh undefine rhel
virsh autostart rhel # (1)

1. Start domain at boot

Inspect a running domain

virsh dominfo $DOMAIN
virsh domiflist $DOMAIN
virsh domblklist $DOMAIN
virsh domrename $DOMAIN $NEWNAME`

Virsh uses XML documents to define domain resources.

virsh dumpxml $DOMAIN
virsh edit $DOMAIN

Several virsh commands come in two varieties, one to handle XML documents that define resources and another to define them inline from the command-line.

XML	CLI	Description
pool-define	pool-define-as	Define a persistent storage pool
vol-create	vol-create-as	Create a volume

The commands ending in -as also support use of the --print-xml option, which will output the equivalent XML document to stdout.

Glossary

Boxes

Boxes is a GNOME Desktop Environment application intended as a more user-friendly alternative to virt-manager to make virtualization easier for end-users.

domain

In libvirt, a domain is a guest VM.

KVM

KVM is a FreeBSD and Linux kernel module that allows the kernel to function as a hypervisor.

KVM was first merged into kernel 2.6.20.

libvirt

libvirt is an open-source API, daemon, and management tool for managing virtualization. It is a C toolkit to interact with the virtualization capabilities of the Linux KVM module, but it can also be used with KVM along with other virtualization technologies like QEMU.

QEMU

QEMU is an open-source emulator that interoperates with KVM to run VMs at near-native speed when the guest architecture is the same as that of the host.

storage pool

In libvirt, a storage pool is a file, directory, or storage device managed by libvirt to provide storage for domains.