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SmartOS is a 100% Live OS. Whether you boot it from an ISO image, USB Key, or netboot (PXE) image, you are essentially loading the OS into memory and running it off a ramdisk. During initial setup, the OS creates a Zpool for persistent storage, but not all of the OS is persistent.

In this topic, you will learn about the advantages of a "diskless" architecture, what is and is not persistent in SmartOS, and how to manage the global zone.

See this topic for a breakdown of the global zone and non-global zones in SmartOS.

Why Diskless Booting is Good

This video explains why and how SmartOS boots from a USB thumb drive.

This video was originally produced for Joyent's SmartDataCenter product. References to things such as "headnode", "computenode", or authentication against APIs do not apply.

VIDEO: Discussion about diskless booting in SmartOS between Bryan Cantrill and Josh Wilsdon of Joyent

Global Zone Persistence

Diskless booting of a hypervisor provides many advantages but enforces contraints on customization and management of the global zone. For example, changes you make in some key root file systems, such as /etc and /usr, will not persist across reboots. The default file system configuration will look like the following (pseudo-filesystems removed for clarity):

The configuration breaks down in the following way:

  • /: The root filesystem is a ramdisk which you should write to sparingly under normal circumstances. All files written here are non-persistant. This includes /root (the root user home directory) and /etc with exception (see below).
  • /usr: This filesystem is a loopback filesystem (located in /usr.lgz) mounted read-only. This includes /usr/local which is commonly used on other OS's such as Linux (see below).
  • /var, /opt: Both these filesystems are persistant and created on the zpool during initial setup. Any data written here is safe.
    Contrary to many Linux distros, /var/run is tmpfs but /var/tmp is not. For temp files you want deleted on reboot, use /tmp or /var/run.
  • /usbkey: This filesystem is a persistant configuration which is applied on boot. It includes a small configuration file (config), a shadow file linked to /etc/shadow, and SSH directory linked to /etc/ssh.
For a good breakdown of changes you can make in the global zone, see this blog post by Jonathan Perkin of Joyent.

Creating Persistent Services Using SMF

On boot, SMF will look for and load any SMF manifests found in /opt/custom/smf. This is the primary way by which you can introduce persistent customization to SmartOS. While we will not provide a complete SMF tutorial here, lets review the basics to get you started.

The Service Management Facility (SMF) is an init replacement for managing userland processes after the OS kernel boots. SMF manages these processes as services, creating contracts with them, and will restart them in the event that they terminate unexpectedly. SMF services are described in XML manifests which describe service dependencies, metadata, and contain methods to start, stop and restart the service. A method is a program or script which forks long running processes which will be managed by SMF. Services which do not spawn long-running processes are known as transient services. For instance, an SMF Service that would set the system hostname would be a "transient service" which executes a start method script which contains the "hostname" command.

Creating a Transient Service

Transient SMF services are particularly useful on SmartOS as a way to simplify customization.

The following is an example of a transient SMF manifest that you would place in /opt/custom/smf to do basic customization:

Walking briefly through this XML:

  1. Starts by defining a service named "smartos/setup". This service is dependent on the "network/physical" and "filesystem/local" services. This means it will not execute until networking is setup and filesystems are mounted.
  2. The "start" and "stop" methods are scripts which pass the method name "start" or "stop" as an argument.
  3. The "transient" duration property is defined indicating that this service will execute and terminate and should not restart automatically.
  4. Finally, the manifest specifies the common name of the service and notes its stability as "unstable".
    this has no real meaning, its simply an old convention indicating it may change in the future.

The method scripts that would correspond to the above manifest might look like this:

You may recognize the above script as very similar to a traditional init script, it simply includes some SMF conventions.

If you added the above manifest to /opt/custom/smf, the method script would execute on each system boot. Simply adding all customization into a script like this and executing it on boot through an SMF transient service is the simplest and most straight forward way to add persistent configuration to your SmartOS system.

Creating an SMF Service

A traditional SMF service will execute a method which forks long running processes. A common SmartOS global zone example would be a monitoring agent. For more detailed information regarding SMF in general, please refer to the "Using the Service Management Facility" section.

Using Configuration Management

While system configuration can be accomplished by means of several SMF services, it is recommended that a configuration management (CM) tool, such as CFengine, Puppet or Chef is utilized instead. This greatly simplifies system management, as a single SMF can be added to execute your CM tool which then handles the rest of the setup itself.

Configuration Management is strongly encouraged! For additional details on the tool of your choice please refer to the Configuration Management on SmartOS section.

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