Image building tools for OpenStack

These tools are the components of TripleO (https://wiki.openstack.org/wiki/TripleO) that are responsible for building disk images.

This repository has the core functionality for building disk images, file system images and ramdisk images for use with OpenStack (both virtual and bare metal). The core functionality includes the various operating system specific modules for disk/filesystem images, and deployment and hardware inventory ramdisks.

The TripleO project also develops elements that can be used to deploy OpenStack itself. These live in the TripleO elements repository (https://git.openstack.org/cgit/openstack/tripleo-image-elements).

What tools are there?

• disk-image-create [-a i386|amd64|armhf] -o filename {element} [{element} ...] Create an image of element {element}, optionally mixing in other elements. Element dependencies are automatically included. Support for other architectures depends on your environment being able to run binaries of that platform. For instance, to enable armhf on Ubuntu install the qemu-user-static package.

• ramdisk-image-create -o filename {element} [{element} ...] : Create a kernel+ ramdisk pair for running maintenance on bare metal machines (deployment, inventory, burnin etc).

  ramdisk-image-create -o deploy.ramdisk deploy

• disk-image-get-kernel filename : DEPRECATED Extract the appropriate kernel and ramdisk to use when doing PXE boot using filename as the image for a machine. Consider using the baremetal element, rather than this tool.

• elements can be found in the top level elements directory.

• element-info : Extract information about elements.

Why?

Automation: While users and operators can manually script or put together ram disks and disk images, mature automation makes customisation and testing easier.

Installation

• Clone the repository locally, then add bin to your path.

• Make sure you have qemu-img (qemu-utils package on Ubuntu/Debian, qemu on Fedora/RHEL) and kpartx installed.

Invocation

The scripts can generally just be run. Options can be set on the command line or by exporting variables to override those present in lib/img-defaults. -h to get help. The image building scripts expect to be able to invoke commands with sudo, so if you want them to run non-interactively, you should either run them as root, with sudo -E, or allow your build user to run any sudo command without password.

Using the variable ELEMENTS_PATH will allow to specify multiple elements locations. It's a colon (:) separated path list, and it will work in a first path/element found, first served approach. The included elements tree is used when no path is supplied, and is added to the end of the path if a path is supplied.

Requirements

If you have 4GB of available physical RAM*, or more, diskimage-builder will create a tmpfs mount to build the image in. This will improve image build time by building in RAM. This can be disabled completely by passing --no-tmpfs to disk-image-create. ramdisk-image-create does not use a tmpfs mount. If tmpfs is not used, you will need enough room in /tmp to store two uncompressed cloud images. If you do have tmpfs, you will still need /tmp space for one uncompressed cloud image and about 20% of that for working files.

* As reported by /proc/meminfo MemTotal

Caches and offline mode

Since retrieving and transforming operating system image files, git repositories, Python or Ruby packages, and so on can be a significant overhead, we cache many of the inputs to the build process in ~/.cache/image-create/. The writing an element documention describes the interface within disk-image-builder for caching. When invoking disk-image-builder the --offline option will instruct disk-image-builder to not refresh cached resources.

Note that we don't maintain operating system package caches, instead depending on your local infrastructure (e.g. Squid cache, or an APT or Yum proxy) to facilitate caching of that layer, so you need to arrange independently for offline mode.

Base images

These are cached by the standard elements - fedora, redhat, ubuntu.

source-repositories

Git repositories and tarballs obtained via the source-repositories element will be cached.

C and C++ compilation

Ccache is configured by the base element. Any compilation that honours ccache will be cached.

PyPI

The pypi element will bind mount a PyPI mirror from the cache dir and configure pip and easy-install to use it.

Design

Images are built using a chroot and bind mounted /proc /sys and /dev. The goal of the image building process is to produce blank slate machines that have all the necessary bits to fulfill a specific purpose in the running of an OpenStack cloud: e.g. a nova-compute node. Images produce either a filesystem image with a label of cloudimg-rootfs, or can be customised to produce whole disk images (but will still contain a filesystem labelled cloudimg-rootfs). Once the file system tree is assembled a loopback device with filesystem (or partition table and file system) is created and the tree copied into it. The file system created is an ext4 filesystem just large enough to hold the file system tree and can be resized up to 1PB in size.

An element is a particular set of code that alters how the image is built, or runs within the chroot to prepare the image. E.g. the local-config element copies in the http proxy and ssh keys of the user running the image build process into the image, whereas the vm element makes the image build a regular VM image with partition table and installed grub boot sector. The mellanox element adds support for mellanox infiniband hardware to both the deploy ramdisk and the built images.

Images must specify a base distribution image element. Currently base distribution elements exist for fedora, rhel, and ubuntu. Other distributions may be added in future, the infrastructure deliberately makes few assumptions about the exact operating system in use. The base image has opensshd running (a new key generated on first boot) and accepts keys via the cloud metadata service, loading them into the distribution specific default user account.

The goal of a built image is to have any global configuration ready to roll, but nothing that ties it to a specific cloud instance: images should be able to be dropped into a test cloud and validated, and then deployed into a production cloud (usually via bare metal nova) for production use. As such, the image contents can be modelled as three distinct portions:

• global content: the actual code, kernel, always-applicable config (like disabling password authentication to sshd).
• metadata / config management provided configuration: user ssh keys, network address and routes, configuration management server location and public key, credentials to access other servers in the cloud. These are typically refreshed on every boot.
• persistent state: sshd server key, database contents, swift storage areas, nova instance disk images, disk image cache. These would typically be stored on a dedicated partition and not overwritten when re-deploying the image.

The goal of the image building tools is to create machine images that contain the correct global content and are ready for 'last-mile' configuration by the nova metadata API, after which a configuration management system can take over (until the next deploy, when it all starts over from scratch).

Existing elements

Elements are found in the subdirectory elements. Each element is in a directory named after the element itself. Elements should have a README.md in the root of the element directory describing what it is for.

Writing an element

Conform to the following conventions:

• Use the environment for overridable defaults, prefixing environment variable names with "DIB_". For example: DIB_MYDEFAULT=${DIB_MYDEFAULT:-default} If you do not use the DIB_ prefix you may find that your overrides are discarded as the build environment is sanitised.

• Consider that your element co-exists with many others and try to guard against undefined behaviours. Some examples:

  • Two elements use the source-repositories element, but use the same filename for the source-repositories config file. Files such as these (and indeed the scripts in the various .d directories listed below) should be named such that they are unique. If they are not unique, when the combined tree is created by disk-image-builder for injecting into the build environment, one of the files will be overwritten.

  • Two elements copy different scripts into /usr/local/bin with the same name. If they both use set -e and cp -n then the conflict will be caught and cause the build to fail.

• If your element mounts anything into the image build tree ($TMP_BUILD_DIR) then it will be automatically unmounted when the build tree is unmounted - and not remounted into the filesystem image - if the mount point is needed again, your element will need to remount it at that point.

Phase Subdirectories

Make as many of the following subdirectories as you need, depending on what part of the process you need to customise. The subdirectories are executed in the order given here. Scripts within the subdirectories should be named with a two-digit numeric prefix, and are executed in numeric order.

• root.d: Create or adapt the initial root filesystem content. This is where alternative distribution support is added, or customisations such as building on an existing image.

  Only one element can use this at a time unless particular care is taken not to blindly overwrite but instead to adapt the context extracted by other elements.

• runs: outside chroot

• inputs: $ARCH=i386|amd64|armhf $TARGET_ROOT=/path/to/target/workarea

• extra-data.d: pull in extra data from the host environment that hooks may need during image creation. This should copy any data (such as SSH keys, http proxy settings and the like) somewhere under $TMP_HOOKS_PATH.

• runs: outside chroot

• inputs: $TMP_HOOKS_PATH

• outputs: None

• pre-install.d: Run code in the chroot before customisation or packages are installed. A good place to add apt repositories.

• runs: in chroot

• install.d: Runs after pre-install.d in the chroot. This is a good place to install packages, chain into configuration management tools or do other image specific operations.

• runs: in chroot

• post-install.d: Run code in the chroot. This is a good place to perform tasks you want to handle after the OS/application install but before the first boot of the image. Some examples of use would be: Run chkconfig to disable unneeded services and clean the cache left by the package manager to reduce the size of the image.

• runs: in chroot

• block-device.d: customise the block device that the image will be made on (e.g. to make partitions). Runs after the target tree has been fully populated but before the cleanup hook runs.

• runs: outside chroot

• inputs: $IMAGE_BLOCK_DEVICE={path} $TARGET_ROOT={path}

• outputs: $IMAGE_BLOCK_DEVICE={path}

• finalise.d: Perform final tuning of the root filesystem. Runs in a chroot after the root filesystem content has been copied into the mounted filesystem: this is an appropriate place to reset SELinux metadata, install grub bootloaders and so on. Because this happens inside the final image, it is important to limit operations here to only those necessary to affect the filesystem metadata and image itself. For most operations, post-install.d is preferred.

• runs: in chroot

• cleanup.d: Perform cleanup of the root filesystem content. For instance, temporary settings to use the image build environment HTTP proxy are removed here in the dpkg element.

• runs: outside chroot

• inputs: $ARCH=i386|amd64|armhf $TARGET_ROOT=/path/to/target/workarea

Environment Variables

To set environment variables for other hooks, add a file to environment.d. This directory contains bash script snippets that are sourced before running scripts in each phase.

Dependencies

Each element has a file named element-deps: a plain text, newline separated list of elements which will be added to the list of elements built into the image at image creation time.

First-boot files

• first-boot.d: DEPRECATED Runs inside the image before rc.local. Scripts from here are good for doing per-instance configuration based on cloud metadata. This will be removed in a future release of diskimage-builder. The os-refresh-config element in tripleo-image-elements is recommended as a replacement.

Ramdisk Elements

Ramdisk elements support the following files in their element directories:

• binary-deps.d : text files listing executables required to be fed into the ramdisk. These need to be present in $PATH in the build chroot (i.e. need to be installed by your elements as described above).

• init.d : POSIX shell script fragments that will be appended to the default script executed as the ramdisk is booted (/init).

• udev.d : udev rules files that will be copied into the ramdisk.

Global image-build variables

• DIB_OFFLINE : this is always set. When not empty, any operations that perform remote data access should avoid it if possible. If not possible the operation should still be attempted as the user may have an external cache able to keep the operation functional.

Structure of an element

The above-mentioned global content can be further broken down in a way that encourages composition of elements and reusability of their components. One possible approach to this would be to label elements as either a "driver", "service", or "config" element. Below are some examples.

• Driver-specific elements should only contain the necessary bits for that driver:

  elements/
     driver-mellanox/
        init           - modprobe line
        install.d/
           10-mlx      - package installation
  

• An element that installs and configures Nova might be a bit more complex, containing several scripts across several phases:

  elements/
     service-nova/
        source-repository-nova - register a source repository
        pre-install.d/
           50-my-ppa           - add a PPA
        install.d/
           10-user             - common Nova user accts
           50-my-pack          - install packages from my PPA
           60-nova             - install nova and some dependencies
        first-boot.d/
           60-nova             - do some post-install config for nova
  

• In the general case, configuration should probably be handled either by the meta-data service (eg, during first-boot.d) or via normal CM tools (eg, salt). That being said, it may occasionally be desirable to create a set of elements which express a distinct configuration of the same software components. For example, if one were to bake a region-specific SSL cert into the images deployed in each region, one might express it like this:

  elements/
     config-az1/
        first-boot.d/
           20-ssl      - add the az1 certificate
     config-az2/
        first-boot.d/
           20-ssl      - add the az2 certificate
  

In this way, depending on the hardware and in which availability zone it is to be deployed, an image would be composed of:

• zero or more driver-elements
• one or more service-elements
• zero or more config-elements

It should be noted that this is merely a naming convention to assist in managing elements. Diskimage-builder is not, and should not be, functionally dependent upon specific element names.

diskimage-builder has the ability to retrieve source code for an element and place it into a directory on the target image during the extra-data phase. The default location/branch can then be overridden by the process running diskimage-builder, making it possible to use the same element to track more then one branch of a git repository or to get source for a local cache. See elements/source-repositories/README.md for more information.

Debugging elements

The build-time environment and command line arguments are captured by the 'base' element and written to /etc/dib_environment and /etc/dib_arguments inside the image.

Export 'break' to drop to a shell during the image build. Break points can be set either before or after any of the hook points by exporting "break=[before|after]-hook-name". Multiple break points can be specified as a comma-delimited string. Some examples:

• break=before-block-device-size will break before the block device size hooks are called.

• break=after-first-boot,before-pre-install will break after the first-boot hooks and before the pre-install hooks.

• break=after-error will break after an error during a in target hookpoint.

Images are built such that the Linux kernel is instructed not to switch into graphical consoles (i.e. it will not activate KMS). This maximises compatibility with remote console interception hardware, such as HP's iLO. However, you will typicallly only see kernel messages on the console - init daemons (e.g. upstart) will usually be instructed to output to a serial console so nova's console-log command can function. There is an element in the tripleo-image-elements repository called "remove-serial-console" which will force all boot messages to appear on the main console.

Ramdisk images can be debugged at run-time by passing "troubleshoot" as a kernel command line argument, or by pressing "t" when an error is reached. This will spawn a shell on the console (this can be extremely useful when network interfaces or disks are not detected correctly).

Testing Elements

Elements can be tested using python. To create a test:

• Create a directory called 'tests' in the element directory.

• Create an empty file called '__init__.py' to make it into a python package.

• Create your test files as 'test_whatever.py', using regular python test code.

To run all the tests use testr - testr run. To run just some tests provide one or more regex filters - tests matching any of them are run - testr run apt-proxy.

Third party elements

Pending implementation. The idea is to have a search path for elements.

Copyright

Copyright 2012 Hewlett-Packard Development Company, L.P. Copyright (c) 2012 NTT DOCOMO, INC.

All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.