.. ixo1464634136835
.. _block-storage-for-virtual-machines:

==================================
Block Storage for Virtual Machines
==================================

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

.. _block-storage-for-virtual-machines-section-N10022-N1001F-N10001:

-------------------------
Root Disk Storage for VMs
-------------------------

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

.. _block-storage-for-virtual-machines-ul-d1c-j5k-s5:

-   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.

.. note::
    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.

.. _block-storage-for-virtual-machines-section-N100A2-N1001F-N10001:

---------------------------------------
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.

.. xbooklink For more information, see |stor-doc|: :ref:`Working with Local Volume Groups <working-with-local-volume-groups>`.

.. caution::
    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 |prod-os| 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.

.. xbooklink For more information, see OpenStack Configuration and Management: :ref:`Specifying the Storage Type for VM Ephemeral Disks <specifying-the-storage-type-for-vm-ephemeral-disks>`.

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

    .. _block-storage-for-virtual-machines-d29e17:

    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.

..  xbooklink For more information, see :ref:`VM Storage Settings for Migration,
    Resize, or Evacuation <vm-storage-settings-for-migration-resize-or-evacuation>`.