f125a8b892
This change addresses a long-standing issue in rST documentation imported from XML.
That import process added backslash escapes in front of various characters. The three
most common being '(', ')', and '_'.
These instances are removed.
Signed-off-by: Ron Stone <ronald.stone@windriver.com>
Change-Id: Id43a9337ffcd505ccbdf072d7b29afdb5d2c997e
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.. ixo1464634136835
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.. _block-storage-for-virtual-machines:
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==================================
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Block Storage for Virtual Machines
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==================================
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Virtual machines use controller or storage host resources for root and
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ephemeral disk storage.
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.. _block-storage-for-virtual-machines-section-N10022-N1001F-N10001:
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-------------------------
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Root Disk Storage for VMs
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-------------------------
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You can allocate root disk storage for virtual machines using the following:
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.. _block-storage-for-virtual-machines-ul-d1c-j5k-s5:
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- Cinder volumes on controller hosts (backed by small Ceph Cluster) or
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storage hosts (backed by large-scale Ceph).
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- Ephemeral local storage on compute hosts, using image-based instance
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backing.
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- Ephemeral remote storage on controller hosts or storage hosts, backed by
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Ceph.
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The use of Cinder volumes or ephemeral storage is determined by the **Instance
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Boot Source** setting when an instance is launched. Boot from volume results in
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the use of a Cinder volume, while Boot from image results in the use of
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Ephemeral storage.
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.. note::
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On systems with one or more single-disk compute hosts configured with local
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instance backing, the use of Boot from volume for all |VMs| is strongly
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recommended. This helps prevent the use of local Ephemeral storage on these
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hosts.
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On systems without dedicated storage hosts, Cinder-backed persistent storage
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for virtual machines is provided using the small Ceph cluster on controller
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disks.
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On systems with dedicated hosts, Cinder storage is provided using Ceph-backed
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|OSD| disks on high-availability and highly-scalable storage hosts.
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.. _block-storage-for-virtual-machines-section-N100A2-N1001F-N10001:
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---------------------------------------
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Ephemeral and Swap Disk Storage for VMs
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---------------------------------------
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Storage for |VM| Ephemeral and swap disks, and for Ephemeral boot disks if the
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|VM| is launched from an image rather than a volume, is provided using the
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**nova-local** local volume group defined on compute hosts.
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The **nova-local** group provides either local Ephemeral storage using
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|CoW|-image-backed storage resources on compute hosts, or remote Ephemeral
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storage, using Ceph-backed resources on storage hosts. You must configure the
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storage backing type at installation before you can unlock a compute host. The
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default type is image-backed local Ephemeral storage. You can change the
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configuration after installation.
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.. xbooklink For more information, see |stor-doc|: :ref:`Working with Local Volume Groups <working-with-local-volume-groups>`.
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.. caution::
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On a compute node with a single disk, local Ephemeral storage uses the root
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disk. This can adversely affect the disk I/O performance of the host. To
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avoid this, ensure that single-disk compute nodes use remote Ceph-backed
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storage if available. If Ceph storage is not available on the system, or is
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not used for one or more single-disk compute nodes, then you must ensure
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that all VMs on the system are booted from Cinder volumes and do not use
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Ephemeral or swap disks.
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On |prod-os| Simplex or Duplex systems that use a single disk, the same
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consideration applies. Since the disk also provides Cinder support, adverse
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effects on I/O performance can also be expected for VMs booted from Cinder
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volumes.
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The backing type is set individually for each host using the **Instance
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Backing** parameter on the **nova-local** local volume group.
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**Local CoW Image backed**
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This provides local Ephemeral storage using a |CoW| sparse-image-format
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backend, to optimize launch and delete performance.
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**Remote RAW Ceph storage backed**
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This provides remote Ephemeral storage using a Ceph backend on a system
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with storage nodes, to optimize migration capabilities. Ceph backing uses a
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Ceph storage pool configured from the storage host resources.
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You can control whether a |VM| is instantiated with |CoW| or Ceph-backed
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storage by setting a flavor extra specification.
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.. 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>`.
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.. caution::
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Unlike Cinder-based storage, Ephemeral storage does not persist if the
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instance is terminated or the compute node fails.
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.. _block-storage-for-virtual-machines-d29e17:
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In addition, for local Ephemeral storage, migration and resizing support
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depends on the storage backing type specified for the instance, as well as
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the boot source selected at launch.
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The **nova-local** storage type affects migration behavior. Live migration is
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not always supported for |VM| disks using local Ephemeral storage.
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.. xbooklink For more information, see :ref:`VM Storage Settings for Migration,
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Resize, or Evacuation <vm-storage-settings-for-migration-resize-or-evacuation>`.
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