Reference Information › User Interface Reference Guide › Defining Simple Appliances › Defining Volumes

Defining Volumes

Volumes define the set of disk volumes available to the appliance. The types of volumes are as follows:

Instantiable Volumes

Instantiable volumes belong to the appliance class and are copied and migrated with every instance of the catalog class. This is most frequently used for boot volumes containing the OS and writable configuration space, such as \etc or /var/log/temp in Linux appliances. You should not use instantiable volumes or boot volumes to keep persistent state or meaningful user data. Those should be kept on placeholder volumes. For additional information, refer to the Tie Boundary to Interior section.

Important The instantiable volume can be lost between startup of the appliance and will not be preserved on copy and migrate of the application where the appliance resides. An exception to that is a singleton. In a singleton, the instantiable volume is always preserved.

Common Volumes

Common volumes hold read only code and data that applies to the appliance class. These volumes are frequently used to keep large appliance code separated from the boot volume. This ensures that copies of the common volume do not need to be made for the instances of that class.

To provide another layer of configuration assurance, the content of the volume cannot be modified at runtime. For example, you can place Linux appliances in the catalog /usr subdirectory in a common volume to share as read only volume to all instances of this appliance.

In addition, you can reduce the size of the instantiable volume by moving files to the common volumes. You can also use the common volume as a boot volume for appliances that use a read only boot volume, such as the Linux Filer appliance. Most appliances support using a read only boot volume to reduce space; however, it is slightly more complex to use and may preserve log files across re-boots.

Placeholder Volumes

Placeholder volumes are intended for persistent state, appliance-specific content, such as files and databases, as well as fixed content, such as html files for a web server. In addition, you can attach volumes to placeholder volumes. You can use these volumes for content that belongs to the application, rather than the appliance class. Even when the placeholder volume belongs to the application, it is not deleted or replaced when the appliance is copied or updated.

When using an appliance instance, you must configure application volumes into the placeholder volume for the appliance to operate. You should place as few constraints as possible on placeholder volumes and not require any specialized preparation. For example, mysql auto creates a placeholder volume on its database, if it does not exist.

For step by step information on creating volumes, refer to Volumes in the Set Class Boundary section of the Appliance Developer Guide.