Chapter 3. Types of instance storage

The instance disk created to store instance data depends on the boot source that you use to create the instance. The instance disk of an instance that you boot from an image is controlled by the Compute service and deleted when the instance is deleted. The instance disk of an instance that you boot from a volume is a persistent volume provided by the Block Storage service.

You can specify that an ephemeral disk is created for the instance by choosing a flavor that configures an ephemeral disk. This ephemeral storage is an empty additional disk that is available to an instance. This storage value is defined by the instance flavor. The default value is 0, meaning that no secondary ephemeral storage is created.

The ephemeral disk appears in the same way as a plugged-in hard drive or thumb drive. It is available as a block device, which you can check using the lsblk command. You can mount it and use it however you normally use a block device. You cannot preserve or reference that disk beyond the instance it is attached to.

You can specify that a swap disk is created for the instance by choosing a flavor that configures a swap disk. This swap storage is an additional disk that is available to the instance for use as swap space for the running operating system.

A block storage volume is persistent storage that is available to an instance regardless of the state of the running instance. You can attach multiple block devices to an instance, one of which can be a bootable volume.

You can attach a config drive to an instance when it boots. The config drive is presented to the instance as a read-only drive. The instance can mount this drive and read files from it. You can use the config drive as a source for cloud-init information. Config drives are useful when combined with cloud-init for server bootstrapping, and when you want to pass large files to your instances. For example, you can configure cloud-init to automatically mount the config drive and run the setup scripts during the initial instance boot. Config drives are created with the volume label of config-2, and attached to the instance when it boots. The contents of any additional files passed to the config drive are added to the user_data file in the openstack/{version}/ directory of the config drive. cloud-init retrieves the user data from this file.