doc: define boot from volume in the glossary

Define it and also link to the term in a few different docs.

Change-Id: I6333deb2f6e85eba3c92128dab4e4b4d35355603

doc/source/admin/configuration/hypervisor-powervm.rst

@@ -62,6 +62,6 @@ Volume Support
 Volume support is provided for the PowerVM virt driver via Cinder. Currently,
 the only supported volume protocol is `vSCSI`_ Fibre Channel. Attach, detach,
 and extend are the operations supported by the PowerVM vSCSI FC volume adapter.
-Boot from volume is not yet supported.
+:term:`Boot From Volume` is not yet supported.
 
 .. _vSCSI: https://www.ibm.com/support/knowledgecenter/en/POWER8/p8hat/p8hat_virtualscsi.htm

doc/source/admin/manage-volumes.rst

@@ -23,8 +23,8 @@ to the :cinder-doc:`block storage admin guide
 <admin/blockstorage-volume-multiattach.html>` for more details about creating
 multiattach-capable volumes.
 
-Boot from volume and attaching a volume to a server that is not
+:term:`Boot from volume <Boot From Volume>` and attaching a volume to a server
-SHELVED_OFFLOADED is supported. Ultimately the ability to perform
+that is not SHELVED_OFFLOADED is supported. Ultimately the ability to perform
 these actions depends on the compute host and hypervisor driver that
 is being used.
 

doc/source/reference/glossary.rst

@@ -13,6 +13,16 @@ Glossary
 
         For more information, refer to :doc:`/admin/aggregates`.
 
+    Boot From Volume
+        A server that is created with a
+        :doc:`Block Device Mapping </user/block-device-mapping>` with
+        ``boot_index=0`` and ``destination_type=volume``. The root volume can
+        already exist when the server is created or be created by the compute
+        service as part of the server creation. Note that a server can have
+        volumes attached and not be boot-from-volume. A boot from volume server
+        has an empty ("") ``image`` parameter in ``GET /servers/{server_id}``
+        responses.
+
     Cross-Cell Resize
         A resize (or cold migrate) operation where the source and destination
         compute hosts are mapped to different cells. By default, resize and

doc/source/user/block-device-mapping.rst

@@ -229,12 +229,12 @@ FAQs
    root disks with volumes?
 
    No, there is nothing automatic within nova that converts a
-   non-boot-from-volume request to convert the image to a root volume.
+   non-:term:`boot-from-volume <Boot From Volume>` request to convert the
-   Several ideas have been discussed over time which are captured in the
+   image to a root volume. Several ideas have been discussed over time which
-   spec for `volume-backed flavors`_. However, if you wish to force users
+   are captured in the spec for `volume-backed flavors`_. However, if you wish
-   to always create volume-backed servers, you can configure the API service
+   to force users to always create volume-backed servers, you can configure
-   by setting :oslo.config:option:`max_local_block_devices` to 0. This will
+   the API service by setting :oslo.config:option:`max_local_block_devices`
-   result in any non-boot-from-volume server create request to fail with a
+   to 0. This will result in any non-boot-from-volume server create request to
-   400 response.
+   fail with a 400 response.
 
 .. _volume-backed flavors: https://review.opendev.org/511965/

doc/source/user/cellsv2-layout.rst

@@ -391,10 +391,11 @@ Since the aggregates are in the API database and the cell conductor cannot
 access that information, so this will fail. In the future this check could be
 moved to the *nova-api* service such that the availability zone between the
 instance and the volume is checked before we reach the cell, except in the
-case of boot from volume where the *nova-compute* service itself creates the
+case of :term:`boot from volume <Boot From Volume>` where the *nova-compute*
-volume and must tell Cinder in which availability zone to create the volume.
+service itself creates the volume and must tell Cinder in which availability
-Long-term, volume creation during boot from volume should be moved to the
+zone to create the volume. Long-term, volume creation during boot from volume
-top-level superconductor which would eliminate this AZ up-call check problem.
+should be moved to the top-level superconductor which would eliminate this AZ
+up-call check problem.
 
 The sixth is detailed in `bug 1781286`_ and similar to the first issue.
 The issue is that servers created without a specific availability zone

doc/source/user/certificate-validation.rst

@@ -66,7 +66,8 @@ Limitations
   the ``RBD`` backend avoiding calls to download and verify on the compute.
 
 * As of the 18.0.0 Rocky release, trusted image certification validation is
-  not supported with volume-backed (boot from volume) instances. The block
+  not supported with volume-backed
+  (:term:`boot from volume <Boot From Volume>`) instances. The block
   storage service support may be available in a future release:
 
   https://blueprints.launchpad.net/cinder/+spec/certificate-validate