Add libvirt-dev-alias spec
Change-Id: Ibdb34287e04dfbd1358ef4dd592b613dfe14a99c
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Dan Smith
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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====================================
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Move to using Libvirt device aliases
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====================================
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https://blueprints.launchpad.net/nova/+spec/libvirt-dev-alias
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Currently we identify devices in Libvirt guest XML by a variety of methods,
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which differs based on the device type (at least). Libvirt now provides a
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device alias mechanism by which we can tie virtual guest devices to an
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identifier we can use to look them up in a stable and generic way. Nova
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should move to using that, which will increase consistency, decrease some
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complexity, and also work around some issues with our current strategy.
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Problem description
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===================
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Nova currently looks up guest devices in XML for attach/detach and other
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modifications using a variety of methods. For example, disk devices use
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the ``serial`` property to identify them uniquely. However, libvirt and
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qemu do not support setting this property on all disk device types, which
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means Nova cannot use that to look up disk devices in a generic way. Further,
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if we have multiple network interfaces with the same MAC address, using that
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as a unique identifier is not sufficient.
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Example volume attachment::
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<disk type='block' device='disk'>
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<driver name='qemu' type='raw' cache='none' io='native'/>
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<source dev='/dev/sda' index='5'/>
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<backingStore/>
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<target dev='vdb' bus='virtio'/>
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<serial>ada5af06-300e-4d07-931d-3cc2bff8a8a9</serial>
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<alias name='virtio-disk1'>
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<address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
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</disk>
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Use Cases
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---------
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As a developer, I want Nova to be able to manage libvirt guest devices in a
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stable and consistent way.
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As a deployer, I want Nova to support things like SCSI LUN passthrough, which
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does not support setting the device serial in libvirt.
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Proposed change
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===============
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Nova's libvirt driver should move to using the device alias mechanism
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[1]_ for identifying all types of devices that are attach- or
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detach-able. For devices like volumes and network interfaces, the
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volume or port UUID should be used. For other devices, some other
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stable identifier that correlates to something in Nova or another
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service's database is required. Libvirt has specific requirements for
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the format of the alias, which must be followed. However, for most
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devices that use a UUID as the primary identifier, we should be able
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to embed that within the alias.
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This is what the above disk example would look like with a
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nova-specified alias::
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<disk type='block' device='disk'>
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<driver name='qemu' type='raw' cache='none' io='native'/>
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<source dev='/dev/sda' index='5'/>
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<backingStore/>
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<target dev='vdb' bus='virtio'/>
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<serial>ada5af06-300e-4d07-931d-3cc2bff8a8a9</serial>
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<alias name='ua-ada5af06-300e-4d07-931d-3cc2bff8a8a9'/>
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<address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
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</disk>
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Alternatives
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------------
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We could keep what we have and continue to not support disk devices that do not
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support using ``serial``.
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We could maintain our own mapping in our database for those device types.
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Data model impact
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-----------------
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Nova's own data model is not affected by this and this is limited to
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nova-compute and the libvirt driver. However, the libvirt XML data that we
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currently maintain will need to change (and existing instances migrated) to
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set the device aliases accordingly.
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REST API impact
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---------------
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None.
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Security impact
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---------------
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None.
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Notifications impact
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--------------------
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None.
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Other end user impact
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---------------------
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End users can currently request SCSI LUN-based disk device mapping, but it does
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not work because we are unable to specify the device serial in that
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configuration. After this change, that existing mechanism will begin to work.
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Performance Impact
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------------------
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No major performance impact to Nova itself, although looking up
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devices by alias will be easier and less computationally
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intense. Further a detach-by-alias routine [2]_ is provided by
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libvirt which may be significantly easier than what we currently need
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to do by generating and providing an XML blob for detach.
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Other deployer impact
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---------------------
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None.
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Developer impact
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----------------
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The libvirt driver will ultimately be simpler after this change.
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Upgrade impact
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--------------
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The only upgrade impact comes from migrating existing instance XML documents
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to specify the device alias. Because we may be migrating instances to/from
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older nodes, we should retain compatibility with alias-less XMLs for some time
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to come.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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dansmith
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Other contributors:
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- kashyap
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- sean-k-mooney
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Feature Liaison
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---------------
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dansmith
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Work Items
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----------
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- Enable setting and parsing the device alias on disk, interface, and pci
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devices
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- Actually set those device aliases in the various parts of the driver that
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create those configs
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- Make the code that looks up devices by device-specific identifiers prefer the
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alias and fall back to the old way
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- Migrate existing instance XMLs on startup when device aliases are missing
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Dependencies
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============
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* Libvirt 3.9.0: https://libvirt.org/formatdomain.html#devices
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Testing
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=======
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Existing devstack jobs should provide sufficient coverage other than the unit
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and functional coverage that will be added. Potentially enabling (and using)
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the LUN passthrough attachment mechanism would be beneficial, but that is
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somewhat beyond the scope of this effort which is just changing the enumeration
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behavior.
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Documentation Impact
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====================
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There really is not much in the way of documentation impact because this
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should be transparent to the operators and users.
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References
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==========
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.. [1] Libvirt's device XML specification: https://libvirt.org/formatdomain.html#devices
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.. [2] Libvirt's detach-by-alias function: https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainDetachDeviceAlias
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History
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=======
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.. list-table:: Revisions
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:header-rows: 1
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* - Release Name
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- Description
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* - 2024.1
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- Introduced
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