Unikernel Devel

Unikernels on clouds? Just a mirage


Sorry about the jokey subject line.

This is both an FYI and a request - the request to MirageOS is at the bottom of this wall of text.

I’ve been doing a fair amount of work to try to get all sorts of things to boot on some of the major cloud platforms.

For better or worse I have learned hard lessons about how things work and what does and does not work. I thought some folks might be interested to hear what I learned.

One of my goals was to get Unikernels on HVM working booting on the various clouds, without containers. I only considered HVM servers, no paravirtualised because you have no control over the kernel with paravirtualised.

Here’s the hard won knowledge, 60 seconds for you to absorb - much pain and time for me to gain. Correct me if I’m wrong.

***** AWS/EC2 -
EC2 uses straightforward DHCP and is super easy to get going from a networking perspective. When the kernel boots up it just needs to request an address via DHCP and it should be good to go.
In most cases the primary hard disk name is xvda. Some operating systems will name the primary disk hda.
AWS gives you screen shots of the graphical console which is somewhat helpful and better than being blind to the boot messages.
AWS also gives delayed access to the serial console output which is very helpful.
AWS runs Xen.

***** Google GCE -
This is a bit easy and a bit hard. GCE uses DHCP. BUT - and it is a big but - the DHCP client MUST implement DHCP option 121 - classless static routes. Whilst it is “standard” behaviour, it is not common. Without implementing option 121 your machine won’t get the static route that is needed for the GCE networking to work correctly. dhclient can do it with some configuration fiddling, I was not successful in getting other dhcp clients such as udhcpc to do option 121 properly. You can’t hard code the static route because it changes depending on the GCE network that the machine came up on, so your dhcp client must correctly implement option 121. Well, you can hard code the static route but that is not a general solution.
The hard disk name on GCE is sda
GCE gives very good serial console output. It does not give access to the graphical console.
GCE runs KVM.

***** Digital Ocean
Digital Ocean droplets have static IP addresses that are injected into the machines /etc/network/interfaces when it boots. You can also get the IP address information by querying the HTTP metadata server via the link local address https://en.wikipedia.org/wiki/Link-local_address but this requires parsing of the returned data to get out the bits you need to configure the interface.
The hard disk name on DO is vda
The magnificent thing about Digital Ocean is that it is the only cloud provider that I have worked with that provides a working, interactive KVM console that displays all the boot messages, which makes diagnostics about a million % easier.
As far as I can tell it does not give serial console output.
Digital Ocean runs KVM.

***** Rackspace
Well I had it working but Rackspace cut off my developer account after 12 months and I can’t get anyone at Rackspace to give me more access to their cloud, despite only ever using < $10 a month of their resources for testing their machines.
Boo Rackspace! I can’t at this stage provide any useful information about how Rackspace networking actually works.
Rackspace hard disk name is xvda
Rackspace runs Xen.

***** Softlayer
After MUCH time and pain I found that the Softlayer cloud is so locked down that you literally cannot control the boot process. Actually let me qualify that - it MIGHT be possible to control the boot process, but since it is impossible to see either serial or graphical boot console then there’s just no way to work out why your kernel is not booting.
If anyone is interested, Softlayer does static IP addressing, with the static address configuration being injected into the OS at boot time via /etc/network/interfaces.
Soft layer gives a KVM console if you are willing to jump through the hoops of configuring a PPTP VPN to access the KVM console address, but it is not visible during critical parts of the boot process, which didn’t give me the diagnostic information I needed to see why things were not booting.
Softlayer hard disk name is xvda
Softlayer runs Xen.

SO, what does that all mean?

Well, my goal is to be able to boot HVM unikernels directly on the clouds, without containers.

Despite the challenges posed by the various approaches to networking, the solutions are actually pretty simple.

To make a unikernel run, here is what that unikernel would need to implement:

** HVM unikernel on AWS - unikernel must implement DHCP
** HVM unikernel on GCE - unikernel must implement DHCP with option 121
** HVM unikernel on Digital Ocean - unikernel must accept a network interfaces file containing static IP address configuration
** HVM unikernel on Rackspace - boo Rackspace - no help for developers!
** HVM unikernel on Softlayer I don’t think this will happen any time soon, but out of interest it’s static IP addressing injected at boot time

In fact it would be ideal to be able to pass an /etc/network/interfaces file in to a unikernel specifying either dhcp or static IP address configuration. This could be a standard approach regardless of which cloud.

Ideally this would be passed in to the unikernel at boot time in an initramfs file, which would look like the initrd line in the example grub.cfg here, i.e. rootfs.cpio.gz

serial --speed=115200 --word=8 --parity=no --stop=1
terminal_input --append serial
terminal_output --append serial
set timeout=1
menuentry ‘mirage’ {
linux /boot/mirageunikernel root=/dev/ram0 console=ttyS0,115200
initrd /boot/rootfs.cpio.gz

Anyhow that’s my request to not only the mirage project but all unikernel projects including HALVM - the ability to pass /etc/network/interfaces into the kernel at boot time via initramfs

Having had a chat with some of the mirage project folks recently I understand the the container approach is more likely to be getting resources than bare metal and presumably that provides easy ways to configure networking but I thought I’d at least ask.

Hopefully the knowledge here will be valuable to others.




@andrewstuart, Apologies for the delay in this post appearing. It seems it was stuck in a queue that I hadn’t noticed before (and was awaiting action from me). I’ll check the site settings to make sure this doesn’t happen again.