RancherOS 0.4 makes major improvements to user experience and system performance

Gray Calendar Icon Published: October 25, 2015
Gray Calendar Icon Updated: November 13, 2019

Today we are excited to announce a major release of RancherOS. The first release of RancherOS was announced just six months ago. At that time, powering an entire operating system with Docker was a really experimental concept. We had good reason to believe it was a good idea, but honestly we didn’t know how well it would play out and what issues we might encounter. I’m excited to say that it’s worked out great. Having said that, we felt that sometimes RancherOS could be a little difficult to grasp for new users. With RancherOS v0.4 we made a number of major enhancements to improve usability and customization. To do this we made improved support for various storage systems, made it easier to run go binaries, simplified configuration, improved the console experience, introduced a better build system, and reduced the size of the runtime footprint.


RancherOS v0.4 is our biggest release yet, quite literally. It clocks in at 27MB. We realize that would take almost 19 floppy disks and would barely fit on your Intel 286 at home, but we just had to increase the size. The release grew for a couple of key reasons. First, we decided to switch to glibc. More on that topic later. Second, we included a lot more kernel modules and firmware. More and more users are running RancherOS on bare metal and we’ve gotten many requests to support a wider variety of hardware. Despite increasing the overall size, RancherOS now consumes less memory. Before RancherOS would always run completely from memory (initrd) and thus the system images would consume memory. Now RancherOS will transfer all contents to disk to free up the memory. At runtime RancherOS only consumes about 80mb of RAM so it will now comfortably run on a 512mb server. (Note: The `rancheros-install` command still requires 1 GB, but we will be looking to address that in the next release.)

rancheros 0.4.0 screenshotBetter Storage

One of the biggest asks for RancherOS was to support more storage options such asNFS, LVM, ZFS, Gluster, etc. In the previous release of RancherOS, this was quite difficult to do, because these different storage solution all need different user space components. We didn’t have a way for users to run the user space component from a container while still making that container also available to Docker. In v0.4, we have solved this by introducing a concept called the “storage context” of user Docker. With this release bby default, user Docker will use the storage from the console container. This means you can easily switch to the Ubuntu console and then install whatever user space components you want in the console. Additionally, if you want to keep a more tidy system, you can install the user space components in your own dedicated storage container, and then configure user Docker to use that container as your storage context.

Simplified Configuration

RancherOS used to have two ways of configuring the system. One way was rancher.yml and the other was cloud-config. We have eliminated any need for rancher.yml and moved everything to cloud-config. This means there is now just a single way to configure RancherOS.

Switch to glibc

A large part of our increase in size is due to the fact that we have switched to glibc. In the previous release, we were using uclibc which is a very small C library, that is designed for embedded systems. The biggest problem with this is that go binaries mostly assume glibc is available. This means that tools like docker-machine and rancher-compose would not run on the default console. By switching to glibc, this allows users to easily run go binaries (which are very common in the Docker ecosystem).

Customizable User Docker

In RancherOS v0.4 we made it very easy to customize the version of Docker used. The most common use case is that users would want to replace the user docker. All that is require is to just place a custom Docker binary in /usr/local/bin. For a custom system Docker, one needs to build a new ISO using our new build system.


Because everything is a container in RancherOS, User Docker would run in a container and the console would also run in a container. This was sometimes confusing to users. The main confusion came from the fact that since User Docker was in it’s own container, it would see a different host filesystem than the console. A command like “docker run -v /foo:/foo ...” would not work as expected because /foo on the console is different from /foo in the Docker container. We decided to address this by collapsing the Docker and console container into what looks like one context to the user. This was done through our “storage context” concept, but the end result is that Docker behaves a lot more like it would on a traditional Linux distribution. Additionally, the console in RancherOS is ephemeral. This means on reboot a new console container would be created every time. This would often confuse users when they would switch to the Ubuntu or Debian consoles. Doing an \“apt-get install\” would install any package they wanted but then after reboot those packages would be gone. For this reason we have made the Ubuntu and Debian console persistent by default. For those wishing to keep an ephemeral console should stay with the default busybox console.

Build System

We completely revamped the build system and have made it a lot more modular. This will pave the way for our upcoming ports to ARM and i386. Right now, the biggest benefit is that it is now very easy to build RancherOS 0.4 with a custom kernel of your choice.


A couple months back we started the Docker libcompose project. The initial code for libcompose was actually a pull from RancherOS. Over half the code we wrote for RancherOS was for Docker Compose support in Go. All of that code has now moved to the libcompose project, and with v0.4, RancherOS is now based on libcompose.

Support for RancherOS on Amazon ECS

With Rancher 0.4 we also worked closely with Amazon to ensure RancherOS runs well onAmazon Web Services’ Elastic Container Service (ECS). With this work, we’ve tested and verified that RancherOS runs well with ECS and is available to download with the ECS container agent pre-built in an AMI and cloud formation template. You can learn more from anexcellent blog post by the team at Amazon.

Easier Docker Machine integration

Previously we built an ISO specifically for Docker Machine. We have now removed the need for that ISO and now there is only one rancheros.iso that is needed for either Docker Machine or a regular RancherOS install.

Kernel Headers and DKMS

We now make the kernel headers easily available such that people can compile there own custom module. This includes support for DKMS.


We’re very excited about this new release and are excited to see even more people start using RancherOS to run their containers with the absolute minimum OS. You can find instructions on how to download RancherOS 0.4 on our GitHubpage, or join us for our next online meetup in November, during which we’ll be covering this new release as part of the agenda. You can register for the meetup by following the link below.

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