Diff comments:
> diff --git a/doc/rtd/topics/boot.rst b/doc/rtd/topics/boot.rst > new file mode 100644 > index 0000000..693a091 > --- /dev/null > +++ b/doc/rtd/topics/boot.rst > @@ -0,0 +1,109 @@ > +============ > +Boot Stages > +============ > +In order to be able to provide the functionality that it does, cloud-init > +must be integrated into the boot in fairly controlled way. > + > +There are 5 stages. > + > +1. **Generator** > +2. **local** > +3. **network** > +4. **config** > +5. **final** > + > +Generator > +######### > +When booting under systemd, a > +`generator > <https://www.freedesktop.org/software/systemd/man/systemd.generator.html>`_ > +will run that determines if cloud-init.target should be included in the > +boot goals. By default, this generator will enable cloud-init. It will > +not enable cloud-init if either: > + > + * A file exists: ``/etc/cloud/cloud-init.disabled`` > + * The kernel command line as fond in /proc/cmdline contains > ``cloud-init=disabled``. > + When running in a container, the kernel command line is not honored, but > + cloud-init will read an environment variable named ``KERNEL_CMDLINE`` in > + its place. > + > +local > +##### > + * **systemd service**: ``cloud-init-local.service`` > + * **runs**: As soon as / is mounted read-write. > + * **blocks**: as much of boot as possible, *must* block network bringup. > + * **modules**: none > + > +The purpose of the local stage is: > + * locate "local" data sources. > + * apply networking configuration to the system (including "Fallback") > + > +In most cases, this stage does not do much more than that. It finds the > +datasource and renders the described network configuration to disk. *if* a network config is present; think network config is somewhat confusing here as it's entirely optional. We may use a local data source but provide no network config at all. > +It then exits and expects for the continued boot of the operating system > +to bring network configuration up. > + > +If this is a new instance, then any previously existing network config When booting for the first time in an instance > +should be wiped. Because network bringup is blocked, we can avoid any > +previous-instance networking from coming up, and having negative side > +affects such as dhcp hooks or broadcast of an old hostname. We avoid issues with previous-instance which may include network configuration by running before init attempts to bring networking online. > + > +In the event that no local datasource is found, 'Fallback' networking > +is rendered. Cloud-init's fallback networking consists of rendering the > +equivalent to "dhcp on eth0", which was historically the most popular > +mechanism for network configuration of a guest. We might include a reference to baking this "common" configuration into the image which proved to be troubling in the presence of "persistent" nic naming. > + > +**Note**: In the past, local data sources have been only those that were > +available without network (such as 'ConfigDrive'). However, as seen in > +the recent additions to the DigitalOcean datasource, even data sources > +that require a network will operate at this stage. s/will/can > + > +network > +####### > + * **systemd service**: ``cloud-init.service`` > + * **runs**: After local stage and configured networking is up. > + * **blocks**: As much of remaining boot as possible. > + * **modules**: ``init_modules`` > + > +This stage requires networking configuration to be up, as it will fully > +process any user-data that is found. Such processing means that any > +``#include`` operations will be done including http. > + > +This stage runs the ``disk_setup`` and ``mounts`` modules which may partition > +and format disks and configure mount points (such as in /etc/fstab). > +On some clouds such as Azure, this stage will create filesystems to be > +mounted, including ones that have stale (previous instance) references in > +/etc/fstab. As such, entries /etc/fstab other than / should not be processed > +until after this stage is done. > + > +A part-handler will run at this stage, as will boothooks including > +cloud-config ``bootcmd``. The user of this functionality has to be aware > +that the system is in the process of booting when their code runs. > + > + > +config > +###### > + * **systemd service**: ``cloud-config.service`` > + * **runs**: After network stage. > + * **blocks**: None. > + * **modules**: ``config_modules`` > + > +This stage runs config modules only. Modules that do not really have an > +affect on other stages of boot are run here. > + > + > +final > +##### > + * **systemd service**: ``cloud-final.service`` > + * **runs**: As final part of boot (traditional "rc.local") > + * **blocks**: None. > + * **modules**: ``final_modules`` > + > +This stage runs as late in boot as possible. The goal is to provide run > +as if the system had fully booted. Any scripts that a user is accustomed > +to running after logging into a system should run correctly here. > +Things that run here include > + > + * package installations > + * configuration management plugins (puppet, chef, salt-minion) > + * user-scripts (including ``runcmd``). > + -- https://code.launchpad.net/~smoser/cloud-init/+git/cloud-init/+merge/310386 Your team cloud init development team is requested to review the proposed merge of ~smoser/cloud-init:doc-boot-stages into cloud-init:master. _______________________________________________ Mailing list: https://launchpad.net/~cloud-init-dev Post to : cloud-init-dev@lists.launchpad.net Unsubscribe : https://launchpad.net/~cloud-init-dev More help : https://help.launchpad.net/ListHelp
