The DXC Blogs – Unikernels


Originally published internally 26 Jan 2016:

Last week Docker Inc acquired Cambridge based Unikernel System Ltd, which has got a lot of people asking ‘what’s a unikernel?’, a question that’s well covered in the linked piece. Going back a few years I covered the launch of Mirage OS, which is the basis for what Unikernel Systems do – they’ve since interfaced it with the Docker API so that unikernels could be managed as if they’re containers.

The acquisition caused Joyent’s CTO Bryan Cantrill to write that Unikernels are unfit for production, where he restates some points that he made when I interviewed him at QCon SF in November. Bryan makes a good point about debugging, but I think there are cases where Unikernels don’t really need to be debugged (and Bryan pretty much made the point when talking about ‘correct software’ when we spoke), which is the essence of ‘Refereeing the Unikernels Slamdown‘.

It’s worth noting that DXC Technology has a (very specialised) dog in this fight with our open source Hanlon project it’s not actually a unikernel (as it works with a regular Linux kernel), but it might be argued that it’s on the unikernel spectrum (and for further exploration of that space take a look at some of the presentations from on topics like rump kernels)


Unikernels haven’t taken over the world, but they’re usefully doing the ‘correct software’ job in things like Docker for Mac. The recent release of LinuxKit also shows that Docker Inc is investing in other places along the ‘unikernel spectrum’ that I referred to, making it easy to build stripped down containers that sit on top of Linux, but aren’t strictly unikernels.

Original Comments


There are number of slides and videos around Unikernels just posted from Docker at SCALE-14 (Lunix Meetup) posted at Recap: Docker at SCALE 14x | Docker Blog


In my view, Chris, the Hanlon-Microkernel project is an example of a Docker container that we deploy (and run) dynamically during the process of iPXE-booting perfectly normal (albeit small) Linux kernel. To provide a bit more detail, we use the RancherOS Linux distribution (a Docker-capable Linux kernel that has a total size, for both the kernel image and it’s RAM disk, of approximately 22MB) as our iPXE-boot kernel and dynamically inject the Hanlon-Microkernel Docker container image into that Linux kernel at boot using a cloud-config that is supplied by the Hanlon server.

A Unikernel (from my understanding), is really just a Linux kernel that has been stripped down to the minimal packages and services that are necessary to run a single application.  In my mind, that is quite different from the approach that is taken by RancherOS (or TinyCore Linux) where a “regular” Linux distribution is compressed to boot quickly (and often run in memory).  In those operating systems you typically have all of the same processes available to you (including standard Linux commands and even services like SSH), giving you much greater access to the system if you need to debug something that has gone wrong in that system.  I guess you could make the argument that it’s in the “unikernel spectrum”, but I tend to think of the approaches taken by Unikernels as being quite different from the approaches we’ve taken for years now to make small kernels (which are typically intended to run multiple services, not just one service).  Just my 0.02 (in your favorite local currency)…


Cantrill was belaboring the use case with points that would fit in the early days of VMware.  It evolved though.  As will Unikernel.

Thing is, its in the toolbag now.


‘Unikernels will send us back to the DOS era’ – DTrace guru Bryan Cantrill speaks out • The Register


I enjoyed the 4th paragraph quotes.


Some great stuff from Brendan Gregg on Unikernel Profiling

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