LXC (LinuX Containers) is a OS-level virtualization technology that allows creation and running of multiple isolated Linux virtual environments (VE) on a single control host. These isolation levels or containers can be used to either sandbox specific applications, or to emulate an entirely new host. LXC uses Linux’s cgroups functionality, which was introduced in version 2.6.24 to allow the host CPU to better partition memory allocation into isolation levels called namespaces . Note that a VE is distinct from a virtual machine (VM), as we will see below.
Docker, previously called dotCloud, was started as a side project and only open-sourced in 2013. It is really an extension of LXC’s capabilities. This it achieves using a high-level API that provides a lightweight virtualization solution to run processes in isolation. Docker is developed in the Go language and utilizes LXC, cgroups, and the Linux kernel itself. Since it’s based on LXC, a Docker container does not include a separate operating system; instead it relies on the operating system’s own functionality as provided by the underlying infrastructure. So Docker acts as a portable container engine, packaging the application and all its dependencies in a virtual container that can run on any Linux server.
VE vs. VM
So what, one may ask, is the difference between these VE’s and a traditional VM? Well, the main difference is that in a VE there is no preloaded emulation manager software as in a VM. In a VE, the application (or OS) is spawned in a container and runs with no added overhead, except for a usually minuscule VE initialization process. There is no hardware emulation, which means that aside from the small memory software penalty, LXC will boast bare metal performance characteristics because it only packages the needed applications. Oh, and the OS is also just another application that can be packaged too. Contrast this to a VM, which packages the entire OS and machine setup, including hard drive, virtual processors and network interfaces. The resulting bloated mass usually takes a long time to boot and consumes a lot of CPU and RAM.
Advantage: VE. So why haven’t VM’s already have gone the way of the dinosaur? The problem with VE’s is that, up to now at least, they cannot be neatly packaged into ready-made and quickly deployable machines – think of the flexibility and time saving offered by Amazon’s myriad AWS machine configs. Also, this means they cannot be easily managed via neat GUI management consoles and they don’t offer some other neat features of VM’s such as IaaS setups and live migration.
So the VE crowd is not unlike the overclockers and modders of the CPU and computer hardware universe – they extract more utility from the standard machine in the market. But doing so calls for advanced technical skills, and results in a highly customized machine that’s not necessarily guaranteed to be inter-operable with others. Also, if you don’t know what you’re doing, you will royally mess up your machine.
What They Do
Think of LXC as supercharged chroot on Linux. It allows you to not only isolate applications, but even the entire OS. Its helper scripts focus on creating containers as lightweight machines - basically servers that boot faster and need less RAM. There are two user-space implementations of containers, each exploiting the same kernel features:
- Libvirt, which allows the use of containers through the LXC driver by connecting to 'lxc:///'. This can be very convenient as it supports the same usage as its other drivers.
- Another implementation, called simply 'LXC', is not compatible with libvirt, but is more flexible with more userspace tools. It is possible to switch between the two, though there are peculiarities which can cause confusion.
Docker, on the other hand, can do much more than this. Docker can offer the following capabilities:
- Portable deployment across machines: you can use Docker to create a single object containing all your bundled applications. This object can then be transferred and quickly installed onto any other Docker-enabled Linux host.
- Versioning: Docker includes git-like capabilities for tracking successive versions of a container, inspecting the diff between versions, committing new versions, rolling back etc.
- Component reuse: Docker allows building or stacking of already created packages. For instance, if you need to create several machines that all require Apache and MySQL database, you can create a ‘base image’ containing these two items, then build and create new machines using these already installed.
- Shared libraries: There is already a public registry (http://index.docker.io/ ) where thousands have already uploaded the useful containers they have created. Again, think of the AWS common pool of different configs and distros – this is very similar.
For a great list of Docker’s capabilities, see this thread on Stackoverflow: http://stackoverflow.com/questions/17989306/what-does-docker-add-to-just-plain-lxc
LXC offers the advantages of a VE on Linux, mainly the ability to isolate your own private workloads from one another. It is a cheaper and faster solution to implement than a VM, but doing so requires a bit of extra learning and expertise.
Docker is a significant improvement of LXC’s capabilities. Its obvious advantages are gaining Docker a growing following of adherents. In fact, it starts getting dangerously close to negating the advantage of VM’s over VE’s because of its ability to quickly and easily transfer and replicate any Docker-created packages. Indeed, it is not a stretch to imagine that VM providers such as Cisco and VMware may already be glancing nervously at Docker – an open source startup that could seriously erode their VM profit margins. If so, we may soon see such providers also develop their own commercial VE offerings, perhaps targeted at large organizations as VM-lite solutions. As they say, if you can’t beat ‘em, commercially join ‘em.
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