Block Storage for Virtual Machines

Virtual machines use controller or storage host resources for root and ephemeral disk storage.

Root Disk Storage for VMs

You can allocate root disk storage for virtual machines using the following:

  • Cinder volumes on controller hosts (backed by small Ceph Cluster) or storage hosts (backed by large-scale Ceph).

  • Ephemeral local storage on compute hosts, using image-based instance backing.

  • Ephemeral remote storage on controller hosts or storage hosts, backed by Ceph.

The use of Cinder volumes or ephemeral storage is determined by the Instance Boot Source setting when an instance is launched. Boot from volume results in the use of a Cinder volume, while Boot from image results in the use of Ephemeral storage.


On systems with one or more single-disk compute hosts configured with local instance backing, the use of Boot from volume for all VMs is strongly recommended. This helps prevent the use of local Ephemeral storage on these hosts.

On systems without dedicated storage hosts, Cinder-backed persistent storage for virtual machines is provided using the small Ceph cluster on controller disks.

On systems with dedicated hosts, Cinder storage is provided using Ceph-backed OSD disks on high-availability and highly-scalable storage hosts.

Ephemeral and Swap Disk Storage for VMs

Storage for VM Ephemeral and swap disks, and for Ephemeral boot disks if the VM is launched from an image rather than a volume, is provided using the nova-local local volume group defined on compute hosts.

The nova-local group provides either local Ephemeral storage using CoW-image-backed storage resources on compute hosts, or remote Ephemeral storage, using Ceph-backed resources on storage hosts. You must configure the storage backing type at installation before you can unlock a compute host. The default type is image-backed local Ephemeral storage. You can change the configuration after installation.


On a compute node with a single disk, local Ephemeral storage uses the root disk. This can adversely affect the disk I/O performance of the host. To avoid this, ensure that single-disk compute nodes use remote Ceph-backed storage if available. If Ceph storage is not available on the system, or is not used for one or more single-disk compute nodes, then you must ensure that all VMs on the system are booted from Cinder volumes and do not use Ephemeral or swap disks.

On StarlingX OpenStack Simplex or Duplex systems that use a single disk, the same consideration applies. Since the disk also provides Cinder support, adverse effects on I/O performance can also be expected for VMs booted from Cinder volumes.

The backing type is set individually for each host using the Instance Backing parameter on the nova-local local volume group.

Local CoW Image backed

This provides local Ephemeral storage using a CoW sparse-image-format backend, to optimize launch and delete performance.

Remote RAW Ceph storage backed

This provides remote Ephemeral storage using a Ceph backend on a system with storage nodes, to optimize migration capabilities. Ceph backing uses a Ceph storage pool configured from the storage host resources.

You can control whether a VM is instantiated with CoW or Ceph-backed storage by setting a flavor extra specification.


Unlike Cinder-based storage, Ephemeral storage does not persist if the instance is terminated or the compute node fails.

In addition, for local Ephemeral storage, migration and resizing support depends on the storage backing type specified for the instance, as well as the boot source selected at launch.

The nova-local storage type affects migration behavior. Live migration is not always supported for VM disks using local Ephemeral storage.