How is Docker different from a normal virtual machine?

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Through this post we are going to draw some lines of differences between VMs and LXCs. Lets first define them.

A virtual machine emulates a physical computing environment, but requests for CPU, memory, hard disk, network and other hardware resources are managed by a virtualization layer which translates these requests to the underlying physical hardware.

In this context the VM is called as the Guest while the environment it runs on is called the Host

Linux Containers (LXC) are operating system-level capabilities that make it possible to run multiple isolated Linux containers, on one control host (the LXC host). Linux Containers serve as a lightweight alternative to VMs as they don’t require the hypervisors viz. Virtualbox, KVM, Xen etc.

Docker originally used LinuX Containers (LXC), but later switched to runC (formerly known as libcontainer), which runs in the same operating system as its host. This allows it to share a lot of the host operating system resources. Also, it uses a layered filesystem (AuFS) and manages networking.

Containers include the application and all of its dependencies –but share the kernel with other containers, running as isolated processes in user space on the host operating system. Docker containers are not tied to any specific infrastructure: they run on any computer, on any infrastructure, and in any cloud.

Case 1
So, let’s say you have a 1GB container image; if you wanted to use a Full VM, you would need to have 1GB times x number of VMs you want. With docker and AuFS you can share the bulk of the 1GB between all the containers and if you have 1000 containers you still might only have a little over 1GB of space for the containers OS (assuming they are all running the same OS image).

Case 2
A full virtualized system gets its own set of resources allocated to it, and does minimal sharing. You get more isolation, but it is much heavier (requires more resources). With docker you get less isolation, but the containers are lightweight (require fewer resources). So you could easily run thousands of containers on a host, and it won’t even blink.

Case 3
A full virtualized system usually takes minutes to start, whereas docker/LXC/runC containers take seconds, and often even less than a second.

Case 4
If you want full isolation with guaranteed resources, a full VM is the way to go. If you just want to isolate processes from each other and want to run a ton of them on a reasonably sized host, then docker/LXC/runC seems to be the way to go.

Case 5
Deploying a consistent production environment is easier said than done. Even if you use tools like chef and puppet, there are always OS updates and other things that change between hosts and environments.

Docker gives you the ability to snapshot the OS into a shared image, and makes it easy to deploy on other docker hosts. Locally, dev, qa, prod, etc.: all the same image. Sure you can do this with other tools, but not nearly as easily or fast.

Case 6
LIGHTWEIGHT – Containers running on a single machine share the same operating system kernel; they start instantly and use less RAM. Images are constructed from layered filesystems and share common files, making disk usage and image downloads much more efficient.

Whereas, Virtual machines include the application, the necessary binaries and libraries, and an entire guest operating system — all of which can amount to tens of GBs.

Rajesh Kumar
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