Ubuntu Kernels

MAAS supports three types of kernels for its Ubuntu nodes.

  • General availability kernels
  • Hardware enablement kernels
  • Low latency kernels

General availability kernels

The general availability (GA) kernel is based on the generic kernel that originally ships with a new Ubuntu version. However, what fixes that have since entered the package archives get applied depend on what type of stream was chosen when setting up the global image source for MAAS ('daily' or 'releases'). The default is 'daily', which indicates that all such fixes will be applied. The 'releases' stream type will also have fixes applied but on a much less frequent basis.

MAAS denotes a GA kernel like this:

ga-<version> : The GA kernel has the major kernel version of the kernel which the corresponding Ubuntu release shipped with. For example, 'ga-16.04' is based on the 'generic' 4.4 Ubuntu kernel. As per Ubuntu policy, a GA kernel will never have its major version upgraded (until the release itself is upgraded).

Hardware enablement kernels

New hardware gets released all the time and if an Ubuntu host is running an older kernel then that hardware likely won't be supported by it. Ubuntu's response to this is to backport more recent kernels. Doing this effectively enables more hardware. Hence, HWE is an acronym for HardWare Enablement.

Clearly, any kernel improvements and new features are also gained by installing an HWE kernel.

Note: There is the notion of an HWE stack. This refers to the (graphical) X portion (in addition to the kernel) when the Ubuntu host is running a desktop environment. This is not the case with MAAS as nodes are provisioned strictly as non-graphical servers.

Note that these backported/HWE kernels are only available for LTS releases (e.g. Precise, Trusty, Xenial, etc). For example, the first available HWE kernel for Ubuntu 16.04 LTS (Xenial) will be the GA kernel from Ubuntu 16.10 (Yakkety).

In MAAS, HWE kernels are referred to by the notation hwe-<release letter>. So, to install the Yakkety HWE kernel on Xenial the hwe-y kernel is used. By default, when using the web UI, MAAS imports all available HWE kernels along with its generic boot images. So if Trusty images are imported then the following HWE kernels are included: hwe-u, hwe-v, and hwe-w.

See MAAS CLI for how to target specific HWE kernels when selecting install images.

See the LTS Enablement Stack Ubuntu wiki for the latest information on HWE.

Low latency kernels

The low latency kernel is based on the GA kernel, but uses a more aggressive configuration to reduce latency. It is categorized as a soft real-time kernel.

MAAS denotes a low latency kernel in a few ways:

hwe-x-lowlatency : the Xenial low latency HWE kernel for Trusty ga-16.04-lowlatency : the low latency GA kernel for Xenial hwe-16.04-lowlatency : the low latency HWE kernel for Xenial

Note: As time of writing, the last 2 kernels are the same.

Using kernels

The kernel installed on a node during deployment is, by default, the Ubuntu release's native kernel. However, it is possible to tell MAAS to use a different kernel. This can be done in three ways:

  • globally (default minimum kernel)
  • per machine (minimum kernel)
  • per machine during deployment (specific kernel)

Note: MAAS will emit an error if a configured minimum kernel version (or newer) is not available for the machine's Ubuntu release.

See MAAS CLI for how to perform these three configurations from the CLI.

Default minimum kernel

To set the default minimum kernel for all machines visit the 'Settings' page and select a kernel in the 'Default Minimum Kernel Version' field. Don't forget to click 'Save'.

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Machine minimum kernel

To set the minimum kernel on a machine basis, select a machine from the 'Nodes' page and click the Edit button in the 'Machine summary' pane. Then select a kernel in the 'Minimum Kernel' field and 'Save changes'.

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Machine kernel during deployment

To set a specific kernel during deployment, select a machine from the 'Nodes' page and choose Deploy under 'Take action'. Then select a kernel in the 'Default kernel' field. Hit 'Go' to initiate the deployment.

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MAAS verifies that the specified kernel is available for the given Ubuntu release (series) before deploying the node.