Monday, June 29, 2009

UML (User Mode Linux)

In this Blog I will take you to a tour to a new dimension of Virtualization. That is the world of UML.

Many of us have spent time using VMware, VBox, Qemu etc, or debugging on how to start Xen/OpenVZ.
But the truth of above two is, applications like VMware are heavy on system resources and applications like Xen are a bit tricky (kernel should be Xen specific).

One thing I will admit, that I was also amongst the people who have spent a huge amount of time on different Virtualization technologies, until here at Openbravo I was given an opportunity to setup virtual environment on Amazon servers.
* For those who haven't worked with Amazon servers, Amazon servers are like domU of Xen virtual environment.

The aim was to setup an Openbravo instance (which can be made available on demand) on the top of an Amazon instance.
The main problem in working with amazon server is that you don't have access to host (domO) and neither Amazon people allow you to use custom kernel for domU (custom kernel: so that we can build another xen domU over it).
So taking these things in to account we were left with options like VMware but looking upon their load on system we needed something light and usable.
In our hunt for a perfect virtual environment my manager told me about UML. And truly speaking it turned up to be the perfect tool (it matched all our requirements and expectations).

UML works very differently from all other Virtualization techniques, all it needs to work smoothly is uml-utilities package, one kernel (binary script) and a block device(which contains minimal/full OS).
It is like doing chroot to a directory and installing a full OS in that.
It has many other features like
  • Mounting host filesystem.
  • Adding a COW (copy on write) file.
  • etc.

This COW file is used if you want to use same block device (filesystem) for more than one Virtual system. This COW file writes the difference to a separate file (just like a diff patch).

The use of UML gave us one more flexibility, that was using Xen image (pre-installed Openbravo instance) with UML kernel.
And an advantage over Xen (i.e. access over host system is not required in UML).

UML Howto
UML Wiki
Download Page

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