When you need a stemcell for a release, you go to the https://www.bosh.io/stemcells page. While it’s clear which Operating System to choose from (Ubuntu or Cent OS) the next few decisions to make can become less clear as you sort through each of the additional options.
In this article, we’ll help you understand what all the terms mean, so you can make an educated decision for your targeted infrastructure.
A Plethora of Choices
At minimum building a Stemcell combines an Infrastructure-as-a-Service (Iaas) and an Operating System (OS) with a Cloud Provider Interface (CPI).
For example on Azure it’s simply, Azure Hyper-V Ubuntu, (or CentOS), etc.
But what about the other choices out there? What’s the difference between a Light and a non-Light? Does the Light stemcell just work out more?
Let’s dig a bit deeper to find out.
|AWS||Xen, Xen Light, Xen HVM Light||Yes||Yes|
|OpenStack||KVM, KVM (raw)||Yes||Yes|
When we look at the options like this we can see some variations in the CPI versions emerge. Here’s a list of which ones we’ll discuss further.
Light Stemcells work with the targeted infrastructure’s system to allow them to host the image. We can then reference the stemcell for import at runtime.
Download a light stemcell, and open the contents. In the
stemcell.MF we can see the metadata for the
cloud_properties. By using the CPI and this metadata, BOSH will be able to obtain the image remotely from the IaaS.
Describes any IaaS-specific properties needed to import OS image.
Check out this example from the AWS Xen Light stemcell:
cloud_properties: ami: eu-west-1: ami-8b6c05f8 us-west-2: ami-d7549db7 eu-central-1: ami-136c9b7c ap-northeast-1: ami-3a95535b ap-northeast-2: ami-3622e858 us-west-1: ami-b5e3a3d5 sa-east-1: ami-fa148396 ap-southeast-1: ami-e1ce1082 ap-southeast-2: ami-9d6652fe us-east-1: ami-3c2fbc2b
Hardware-assisted virtualization (HVM) Stemcells
AWS EC2 uses Xen to virtualize machine instances. Two of the most common Guest Types used are:
- Paravirtualization (PV)
- Hardware-assisted virtualization (HVM)
Previous generation instances of EC2 were typically Paravirtualized servers, which would be the "AWS Xen Light" choice.
The Amazon Linux AMI Instance Type Matrix is a useful guide to help you determine if the instance size you’re about to use is compatible with the "AWS Xen HVM Light" choice of stemcell.
When the target infrastructure is OpenStack ask yourself if you’ll be using Cinder block storage.
If so, you’d want to use the raw uncompressed stemcell option "OpenStack KVM (raw)". Then configure the
root_disk property to include the
boot_from_volume: true to enable cinder-backed boot volumes.
We hope this has helped to give you some insights into which stemcells are the right tool for the job. Let us know if you have any questions we may have missed when talking about variations of stemcells.