Update driver docs with constraint examples

2016-10-03 17:01:24 -04:00

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layout	page_title	sidebar_current	description
docs	Drivers: Qemu	docs-drivers-qemu	The Qemu task driver is used to run virtual machines using Qemu/KVM.

Qemu Driver

Name: qemu

The Qemu driver provides a generic virtual machine runner. Qemu can utilize the KVM kernel module to utilize hardware virtualization features and provide great performance. Currently the Qemu driver can map a set of ports from the host machine to the guest virtual machine, and provides configuration for resource allocation.

The Qemu driver can execute any regular qemu image (e.g. qcow, img, iso), and is currently invoked with qemu-system-x86_64.

The driver requires the image to be accessible from the Nomad client via the artifact downloader.

Task Configuration

The Qemu driver supports the following configuration in the job spec:

image_path - The path to the downloaded image. In most cases this will just be the name of the image. However, if the supplied artifact is an archive that contains the image in a subfolder, the path will need to be the relative path (subdir/from_archive/my.img).
accelerator - (Optional) The type of accelerator to use in the invocation. If the host machine has Qemu installed with KVM support, users can specify kvm for the accelerator. Default is tcg.
port_map - (Optional) A map[string]int that maps port labels to ports on the guest. This forwards the host port to the guest VM. For example, port_map { db = 6539 } would forward the host port with label db to the guest VM's port 6539.
args - (Optional) A []string that is passed to qemu as command line options. For example, args = [ "-nodefconfig", "-nodefaults" ].

Examples

A simple config block to run a Qemu image:

task "virtual" {
  driver = "qemu"

  config {
    image_path = "local/linux.img"
    accelerator = "kvm"
    args = [ "-nodefaults", "-nodefconfig" ]
  }

  # Specifying an artifact is required with the "qemu"
  # driver. This is the # mechanism to ship the image to be run.
  artifact {
    source = "https://dl.dropboxusercontent.com/u/1234/linux.img"

    options {
      checksum = "md5:123445555555555"
    }
  }

Client Requirements

The Qemu driver requires Qemu to be installed and in your system's $PATH. The task must also specify at least one artifact to download, as this is the only way to retrieve the image being run.

Client Attributes

The Qemu driver will set the following client attributes:

driver.qemu - Set to 1 if Qemu is found on the host node. Nomad determines this by executing qemu-system-x86_64 -version on the host and parsing the output
driver.qemu.version - Version of qemu-system-x86_64, ex: 2.4.0

Here is an example of using these properties in a job file:

job "docs" {
  # Only run this job where the qemu version is higher than 1.2.3.
  constraint {
    attribute = "${driver.qemu.version}"
    operator  = ">"
    value     = "1.2.3"
  }
}

Resource Isolation

Nomad uses Qemu to provide full software virtualization for virtual machine workloads. Nomad can use Qemu KVM's hardware-assisted virtualization to deliver better performance.

Virtualization provides the highest level of isolation for workloads that require additional security, and resource use is constrained by the Qemu hypervisor rather than the host kernel. VM network traffic still flows through the host's interface(s).

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