Building Guest Images for OpenStack Trove


When Trove receives a command to create a guest instance, it does so by launching a Nova instance based on the appropriate guest image that is stored in Glance.

To operate Trove it is vital to have a properly constructed guest image, and while tools are provided that help you build them, the Trove project itself does not distribute guest images. This document shows you how to build guest images for use with Trove.

It is assumed that you have a working OpenStack deployment with the key services like Keystone, Glance, Swift, Cinder, Nova and networking through either Nova Networks or Neutron where you will deploy the guest images. It is also assumed that you have Trove functioning and all the Trove services operating normally. If you don’t have these prerequisites, this document won’t help you get them. Consult the appropriate documentation for installing and configuring OpenStack for that.

High Level Overview of a Trove Guest Instance

At the most basic level, a Trove Guest Instance is a Nova instance launched by Trove in response to a create command. For most of this document, we will confine ourselves to single instance databases; in other words, without the additional complexity of replication or mirroring. Guest instances and Guest images for replicated and mirrored database instances will be addressed specifically in later sections of this document.

This section describes the various components of a Trove Guest Instance.

Operating System and Database

A Trove Guest Instance contains at least a functioning Operating System and the database software that the instance wishes to provide (as a Service). For example, if your chosen operating system is Ubuntu and you wish to deliver MySQL version 5.7, then your guest instance is a Nova instance running the Ubuntu operating system and will have MySQL version 5.7 installed on it.

Trove Guest Agent

Trove supports multiple databases, some of them are relational (RDBMS) and some are non-relational (NoSQL). In order to provide a common management interface to all of these, the Trove Guest Instance has on it a ‘Guest Agent’. The Trove Guest Agent is a component of the Trove system that is specific to the database running on that Guest Instance.

The purpose of the Trove Guest Agent is to implement the Trove Guest Agent API for the specific database. This includes such things as the implementation of the database ‘start’ and ‘stop’ commands. The Trove Guest Agent API is the common API used by Trove to communicate with any guest database, and the Guest Agent is the implementation of that API for the specific database.

The Trove Guest Agent runs inside the Trove Guest Instance.

Injected Configuration for the Guest Agent

When TaskManager launches the guest VM it injects the specific settings for the guest into the VM, into the file /etc/trove/conf.d/guest_info.conf. The file is injected one of three ways.

If use_nova_server_config_drive=True, it is injected via ConfigDrive. Otherwise it is passed to the nova create call as the ‘files’ parameter and will be injected based on the configuration of Nova; the Nova default is to discard the files. If the settings in guest_info.conf are not present on the guest Guest Agent will fail to start up.

Persistent Storage, Networking

The database stores data on persistent storage on Cinder (if configured, see trove.conf and the volume_support parameter) or ephemeral storage on the Nova instance. The database service is accessible over the tenant network provided when creating the database instance.

The cloud administrator is able to config a management networks(CONF.management_networks) that is invisible to the cloud tenants, database instance can talk to the control plane services(e.g. the message queue) via that network.

Building Guest Images using DIB

A Trove Guest Image can be built with any tool that produces an image accepted by Nova. In this document we describe how to build guest images using the ‘Disk Image Builder’ (DIB) tool, and we focus on building qemu images.

DIB uses a chroot’ed environment to construct the image. The goal is to build a bare machine that has all the components required for launch by Nova.

Build image using trovestack

Trove provides a script called trovestack that could do most of the management and test tasks. Refer to trovestack document for the steps to build trove guest agent images.

Disk Image Builder ‘Elements’

DIB Elements are ‘executed’ by the disk-image-create command to produce the guest image. An element consists of a number of bash scripts that are executed by DIB in a specific order to generate the image. You provide the names of the elements that you would like executed, in order, on the command line to disk-image-create.

DIB comes with some built-in elements. In addition, projects like TripleO provide elements as well.

Trove also provides a set of its own elements. In keeping with the philosophy of making elements ‘layered’, Trove provides two sets of elements. The first implements the guest agent for various operating systems and the second implements the database for these operating systems.

Contributing Reference Elements When Implementing a New ‘Datastore’

When contributing a new datastore, you should contribute elements that will allow any user of Trove to be able to build a guest image for that datastore.

Considerations in Building a Guest Image

In building a guest image, there are several considerations that one must take into account. Some of the ones that we have encountered are described below.

Speed of Launch and Start-up Activities

The actions performed on first boot can be very expensive and may impact the time taken to launch a new guest instance. So, for example, guest images that don’t have the database software pre-installed and instead download and install during launch could take longer to launch.

In building a guest image, therefore care should be taken to ensure that activities performed on first boot are traded off against the demands for start-time.

Database licensing, and Database Software Download Issues

Some database software downloads are licensed and manual steps are required in order to obtain the installable software. In other instances, no repositories may be setup to serve images of a particular database. In these cases, it is suggested that an extra step be used to build the guest image.

User Manually Downloads Database Software

The user manually downloads the database software in a suitable format and places it in a specified location on the machine that will be used to build the guest image.

An environment variable ‘DATASTORE_PKG_LOCATION’ is set to point to this location. It can be a single file (for example new_db.deb) or a folder (for example new_db_files) depending on what the elements expect. In the latter case, the folder would need to contain all the files that the elements need in order to install the database software (a folder would typically be used only if more than one file was required).

Use an extra-data.d Folder

Use an extra-data.d folder for the element and copy the file into the image

Steps in extra-data.d are run first, and outside the DIB chroot’ed environment. The step here can copy the installable from DATASTORE_PKG_LOCATION into the image (typically into TMP_HOOKS_PATH).

For example, if DATASTORE_PKG_LOCATION contains the full path to an installation package, an element in this folder could contain the following line:


Use an install.d Step to Install the Software

A standard install.d step can now install the software from TMP_HOOKS_DIR.

For example, an element in this folder could contain:

dpkg -i ${TMP_HOOKS_PATH}/new_db.deb

Once elements have been set up that expect a package to be available, the guest image can be created by executing the following:

DATASTORE_PKG_LOCATION=/path/to/new_db.deb ./

Assuming the elements for new_db are available in the trove repository, this would equate to:

DATASTORE_PKG_LOCATION=/path/to/new_db.deb ./trovestack kick-start new_db