The Excellence of libvirt

Table of Contents

At some point, I needed to use some VMs. At first I used direct invocations of the qemu command-line tool, but the command line arguments were hard to put together, and the whole edifice seemed unstable.

Then I started using virt-manager, which is based on libvirt, and I've been very impressed with the entire libvirt ecosystem.

VMs in a GUI

The virt-manager GUI is very straightforward. I can create, stop, start and destroy my VMs. I can view their graphical consoles. It presents a vast amount of functionality in a pretty straightforward and simple way, and it was easy to grow into as I've expanded my use of libvirt.

If I really wanted something super-simple, I could use GNOME Boxes. It uses libvirt like virt-manager, so all the VMs are shared, but strives for simplicity and intuitiveness. So if I wanted to graduate to a program that exposes more functionality, I could always move from GNOME Boxes to virt-manager.

And they are both native applications, so I don't have to deal with the problems, glitches, and performance issues of a similarly complicated web application. There are web app interfaces to libvirt available, but I've never used any of them.

VMs at the command line

I generally prefer command line interfaces for many reasons. Thus, after some basic use of virt-manager, I looked into its command-line counterpart "virsh". Like virt-manager and GNOME Boxes, it uses libvirt, so has automatic access to all the local VMs.

virsh does not include a command to create a new VM like virt-manager does. This is handled by another command line tool, "virt-install", which is produced by the same project as virt-manager. It is quite capable; it can automatically mount ISOs in VMs for installs, download and install from network URLs, PXE boot, all that stuff. Naturally, virt-manager has all these functions as well.

I should note that both virsh and virt-install connect by default to an unprivileged, not-hardware-accelerated, user-specific "session" hypervisor. Correct this with the argument "–connect qemu:///system" to both, to connect to the KVM-using system hypervisor that virt-manager uses by default.

Now I split my time between the virt-manager GUI and virsh/virt-install on the command line. For the most basic tasks, like restarting a VM, I use virsh. For ones of intermediate complexity, like modifying the configuration of a VM, I use virt-manager. And for the most sophisticated tasks, or scripting, I return to virsh and other command line tools.

VMs remotely

When I needed to remotely manage my VMs, I was again delighted. The use of a command line tool allows me to do remote work with ssh. But in fact, libvirt has remote support built into the library. Just as virt-manager, virsh and GNOME Boxes connect to the local libvirtd, which stores information about the VMs, any libvirt tool can remotely connect to a libvirt daemon over the network, just by passing an extra –connect argument. This can be done in multiple ways. I generally use the (automatically set up) ssh tunnelling, as it doesn't require any configuration on the remote libvirtd.

This means I can even use virt-manager remotely! And I do, frequently; I have the exact same workflow for local VMs as for remote ones. I can connect to all my VM hosts and get a nice dashboard of everything that's going on.

If a VM has a graphical display (provided by the underlying virtualization software) exposed through VNC/SPICE, it doesn't need to listen on anything but the local network interface. virt-manager helpfully reuses the ssh connection to tunnel a connection to the VM's display.

VMs on any hypervisor

I didn't really care about how the VMs were running. I use QEMU/KVM most of the time, but mainly because it's what libvirt uses by default. But libvirt - as a generic API - will even work with the Xen hosts that I deal with.

And if I want to make use of containers, that works too. Though the container support is somewhat barebones, I'm sure it will become more developed as time goes on - container technology as a whole is very raw at the moment. And libvirt containers (like all libvirt virtual machines) are known to systemd-machined (which puts all VMs on the system into their own cgroups, as an extra layer of protection against hypervisor breakouts). Thus systemd can interact with the systemd instance inside the containers just like normal, and provide useful features like "machinectl login CONTAINERNAME".

Ecosystem of VM tools

The ecosystem of tools around libvirt is really cool and really powerful. All of it uses libvirt, so everything gets support for many hypervisors, accessed locally or remotely. This reuse of a single powerful, well developed, mature library is very appealing.

For example, virt-edit (as well as the other libguestfs tools) can edit a file inside a VM's disk image, without having to manually mount it. virt-top gives me a nice terminal overview of the VMs running currently. virt-viewer can serve as a standalone complement to virt-manager, so I can use virsh and virt-install and still see the VM's graphical display.

The libguestfs suite of tools specifically are very powerful and well integrated with libvirt. virt-rescue is a game-changer for repairing VMs. When migrating VMs from Xen to KVM, some simple scripting around guestfish was essential. I use these tools frequently - they make VM work significantly easier.

Conclusion

There are still other features of libvirt that I haven't yet touched. For example, it has the capability to set fine grained access controls on VMs through the use of polkit. It also provides live migration of VMs between hosts.

I find myself in agreement with Gwern's ideas on choosing software, and usually I am suspicous of software that has a wide variety of features, so I feel the need to justify my use of this software. I feel comfortable using libvirt, because of its relatively long life (in these times, 2005 was ages ago), continued widespread use, and large ecosystem. I believe that libvirt is a good level of abstraction1 for VM management.

Footnotes:

1

Inevitably, some people try to put additional layers of abstraction over libvirt, in an attempt to even further "simplify" VM management, or abstract over the datacenter as a whole. These are projects like OpenStack or VirtKick.

I feel these projects are in error. They overcomplicate (vastly, in the case of OpenStack) the internals of managing virtual machines, and they prevent the use of the existing ecosystem of excellent libvirt-based tools, which already makes virtual machine management extremely simple. I suspect one reason for the existence of these projects is that libvirt's client-side tools are not as easily available on proprietary systems like OS X or Windows. If you wish to use an API other than the one libvirt natively provides, you should implement that as a thin layer over libvirt.

Created: 2014-12-21 Sun 15:43

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