openstack/nova
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Add os-volume_attachments reference docs

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This change adds a simple sequence diagram showing the flow of a volume
attachment between the various services, using the libvirt driver as an
example virt driver.

Change-Id: I631ac9de3d48aa0ad849f6615d0ad2052cb63e80
This commit is contained in:
Lee Yarwood 2020-11-02 17:28:26 +00:00
parent edd8fefe3f
commit b62f9a04a0
3 changed files with 71 additions and 0 deletions
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34
doc/source/reference/attach-volume.rst Normal file
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..
Licensed under the Apache License, Version 2.0 (the "License"); you may
not use this file except in compliance with the License. You may obtain
a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
=================
Attaching Volumes
=================
The following sequence diagram outlines the current flow when attaching a
volume to an instance using the ``os-volume_attachments`` API. This diagram
uses the ``libvirt`` driver as an example virt driver to additionally document
the optional interactions with the ``os-brick`` library on the compute hosts
during the request.
.. note:: ``os-brick`` is not always used to connect volumes to the host, most
notibly when connecting an instance natively to ceph ``rbd`` volumes
The diagram also outlines the various locks taken on the compute during the
attach volume flow. In this example these include locks against the
``instance.uuid``, ``cinder_backend.uuid`` orchestrated for ``nova-compute`` by
``os-brick`` and the generic ``connect_volume`` lock taken within os-brick
itself. This final ``connect_volume`` lock also being held when detaching and
disconnecting a volume from the host by ``os-brick``.
.. seqdiag:: attach_volume.diag

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doc/source/reference/attach_volume.diag Normal file
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seqdiag {
user; nova-api; nova-conductor; nova-compute; libvirt-driver; os-brick; cinder-api;
edge_length = 300;
span_height = 16;
activation = none;
default_note_color = white;
user -> nova-api [label = "POST /servers/{server_id}/os-volume_attachments"];
nova-api -> nova-compute [label = "RPC call reserve_block_device_name"];
nova-compute -> nova-compute [label = "instance.uuid lock"];
nova-compute ->> nova-conductor [label = "bdm.create"];
nova-compute <<- nova-conductor [label = "return BlockDeviceMapping"];
nova-compute -> libvirt-driver [label = "get_device_name_for_instance"];
nova-compute <- libvirt-driver [label = "Return get_device_name_for_instance"];
nova-api <- nova-compute [label = "Return reserve_block_device_name"];
nova-api -> cinder-api [label = "POST /v3/{project_id}/attachments"];
nova-api <- cinder-api [label = "Return HTTP 200 (without connection_info)"];
nova-api ->> nova-compute [label = "RPC cast attach_volume"];
user <- nova-api [label = "Return HTTP 200 (includes device_name)"];
nova-compute -> nova-compute [label = "instance.uuid lock"];
nova-compute -> os-brick [label = "cinder_backend.uuid lock"];
nova-compute -> cinder-api [label = "PUT /v3/{project_id}/attachments/{attachment_id}"];
nova-compute <- cinder-api [label = "Return HTTP 200 (includes connection_info)"];
nova-compute -> libvirt-driver [label = "attach_volume"];
libvirt-driver -> os-brick [label = "connect_volume"];
os-brick -> os-brick [label = "connect_volume lock"];
libvirt-driver <- os-brick;
libvirt-driver -> libvirt-driver [label = "guest.attach_device"];
libvirt-driver -> libvirt-driver [label = "_build_device_metadata"];
libvirt-driver ->> nova-conductor [label = "instance.save"];
nova-compute <- libvirt-driver [label = "Return attach_volume"];
nova-compute ->> nova-conductor [label = "bdm.save"];
nova-compute -> cinder-api [label = "POST /v3/{project_id}/attachments/{attachment_id}/action (os-complete)"];
nova-compute <- cinder-api [label = "Return HTTP 200"];
}

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doc/source/reference/index.rst
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@ -37,6 +37,8 @@ The following is a dive into some of the internals in nova.
* :doc:`/reference/isolate-aggregates`: Describes how the placement filter
works in nova to isolate groups of hosts.
* :doc:`/reference/attach-volume`: Describes the attach volume flow, using the
libvirt virt driver as an example.
.. # NOTE(amotoki): toctree needs to be placed at the end of the secion to
# keep the document structure in the PDF doc.
@ -56,6 +58,7 @@ The following is a dive into some of the internals in nova.
conductor
isolate-aggregates
api-microversion-history
attach-volume
Debugging
=========