Virtuozzo

Virtuozzo (previously known as OpenVZ; Figure 3) was also a pioneer in terms of containers for Linux. Although it took far longer than Linux-VServer to establish its market niche, it still works fairly well: OpenVZ Virtuozzo, which had commercial branches later on, soon established a large community following. SWsoft (who later acquired Parallels and changed the company name to reflect the fact) was able to offer users a complete virtualization solution based on Linux for the first time – including tools and kernel patches.

Figure 3: The stable version of OpenVZ still relies on Linux 2.6.32, but Parallels has promised to release a new version based on RHEL 7 in the near future.

If you do set up Virtuozzo in your data center, you can rely on a package that leaves nothing to be desired. Immediately after the install, the distribution asks you if you want to create containers – virtual private servers (VPS) or virtual environments (VEs).

Virtuozzo, like Linux-VServer, originated in age when the Linux kernel did not have the required virtualization infrastructure in place, so a patched kernel is still required for Virtuozzo releases even now.

From a technical point of view, the developers have done some amazing work. CPU and I/O quotas can be defined for containers so that an application running wild in a container does not compromise the host and take down containers operated by other users. Each container is given its own network stack, for which you can define separate firewall rules; however, you do have to configure this explicitly.

Users in a container cannot access any other system resources. Process IDs are virtualized; each container has an init process with an ID of 1. Users in the container do not see which processes are running in VEs operated by other users. The same thing applies to accessing system libraries or the files in /dev, /proc, and /sys.

The Never-Ending Kernel Story

Virtuozzo was a commercial top seller for many years, and the open source version of Virtuozzo and Parallels Cloud Server (as the project was renamed later) shared parts of the same code. The commercial Parallels offering contained several components that were missing from the open source solution.

In 2014, the powers that be decided to merge the free and commercial components and release them as a completely free product in the future. However, this step probably wasn't entirely motivated by management suddenly discovering their passion for free software. After all, maintaining OpenVZ was a labor of Sisyphus for many years.

The reason for all this work is the separate kernel patch, which Virtuozzo, like Linux-VServer, needs. Although the developers started rewriting parts of the required functions and merging them with the Linux kernel – for example, parts of the cgroups implementation were realized under their lead – Virtuozzo still has not fully broken with its custom functions to this day. You still need to patch the current kernel for everything to work.

Parallels Cloud Server still relies on Red Hat Enterprise Linux (RHEL) 6 and comes with Linux 2.6.32, albeit in Red Hat's Frankenstein incarnation. An old kernel like this can fail on state-of-the-art hardware, so Virtuozzo is hardly in the running for new deployments. Today, a beta version at least is based on RHEL 7, which fully complies with FL/OSS rules. Whether or not the vendor today, Odin, will manage to regain lost trust in the product still remains doubtful [3].

LXC

One of the reasons Virtuozzo has a hard row to hoe is that LXC [4] (Figure 4) already provides a complete container solution that can be deployed immediately with any plain vanilla kernel. That it was the OpenVZ developers, with their various patches, who made LXC in its original form possible is possibly one of the greatest ironies of history.

Figure 4: LXC was the first genuine container solution for Linux, but it is not the most successful or the most popular.

LXC containers officially saw the light of day in 2008. At the time, Linux 2.6.24 had just been released. A few kernel releases before this, the functions required for LXC containers had already made their way into the kernel. Cgroups (control groups) is a kernel interface that makes it possible to restrict the privileges and permissions of individual programs on a Linux system. Cgroups originally went by the name "process containers" because they supported operations with individual processes in virtual environments that were screened off from the rest of the system.

The main initiator was Google, but support for the new technology was soon coming in from various directions. Shortly after the release of the first LXC version, the new technology attracted some initial hype. For the first time, admins had a tool for virtualizing processes on a system without needing a complete virtual machine. In test setups, LXC-based solutions were regularly deployed at the time.

However, they rarely found their way from planning into production, because LXC posed a couple of unanswered questions relating to the security of the individual containers. If you had root privileges in one container, you were able – for a long time – to issue arbitrary commands with root privileges on the host. This did not change until Linux kernel 3.8 was released, which offered new cgroups functions for the first time.

In 2013, the kernel developers put cgroups through a low-level redesign, adding namespaces, which created virtual network stacks or PIDs that are strictly segregated from other namespaces or the main system. If a process is running in one network namespace, it only sees the (completely virtual) network interfaces in that namespace.

Users can only indirectly access the network cards the host uses, and they cannot see them at all. Also, users do not see processes that belong to a separate PID namespace that runs processes in other namespaces on the system. Namespaces in combination with cgroups thus offer far stricter segregation of individual containers than was previously the case.

Now, Linux version 3.8 and later had all the tools needed to run containers in a meaningful way. However, LXC never fully recovered from poor marketing. Shortly before it became interesting for corporate users, another tool had entered the stage and conquered the hearts of administrators and hipsters alike: Docker [5].