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第3章 Configuring OpenStack Compute storage

This chapter describes the architecture for the back-end storage of images in OpenStack Compute (nova), and provides basic configuration options.

3.1. Architecture overview

In Red Hat OpenStack Platform, the OpenStack Compute service uses the KVM hypervisor to execute compute workloads. The libvirt driver handles all interactions with KVM, and enables the creation of virtual machines.

Two types of libvirt storage must be considered for Compute:

  • Base image, which is a cached and formatted copy of the Image service image.
  • Instance disk, which is created using the libvirt base and is the back end for the virtual machine instance. Instance disk data can be stored either in Compute’s ephemeral storage (using the libvirt base) or in persistent storage (for example, using Block Storage).

Creation of Virtual Machines

The steps that Compute takes to create a virtual machine instance are:

  1. Cache the Image service’s backing image as the libvirt base.
  2. Convert the base image to the raw format (if configured).
  3. Resize the base image to match the VM’s flavor specifications.
  4. Use the base image to create the libvirt instance disk.

In the diagram above, the #1 instance disk uses ephemeral storage; the #2 disk uses a block-storage volume.

Ephemeral storage is an empty, unformatted, additional disk available to an instance. This storage value is defined by the instance flavor. The value provided by the user must be less than or equal to the ephemeral value defined for the flavor. The default value is 0, meaning no 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 format it, mount it, and use it however you normally would a block device. There is no way to preserve or reference that disk beyond the instance it is attached to.

Block storage volume is persistant storage available to an instance regardless of the state of the running instance.