Allow volume operations when shelved
Volume operations are allowed when an instance is shutdown, paused and even soft deleted, but not when shelved or shelved_offloaded. This spec will enable them when shelved and shelved_offloaded as well. Besides being useful in itself, this is also a first step towards allowing operations on the boot volume when shelved. blueprint volume-ops-when-shelved APIImpact Change-Id: I52f32831bad5f5e1ff378c51186ee798dc3efa2e
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Paul Murray
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specs/mitaka/approved/volume-ops-when-shelved.rst
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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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Volume Operations When Shelved
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==============================
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https://blueprints.launchpad.net/nova/+spec/volume-ops-when-shelved
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Currently attach, detach and swap volume operations are allowed when
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an instance is paused, stopped and soft deleted, but are
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not allowed when an instance has been shelved. These operations are
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possible when an instance is shelved so we should enable them.
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Problem description
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===================
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The attach, detach and swap volume operations are not allowed when an
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instance is in the shelved or shelved_offloaded states. From a user's
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perspective this is at odds with the fact these operations can be
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performed on instances in other inactive states.
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Use Cases
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---------
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As a cloud user I want to be able to detach volumes from my shelved instance
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and use them elsewhere, without having to unshelve the instance first.
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As a cloud user I want to be able to perform all the volume operations on
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a shelved instance that I can when it is stopped, paused or soft deleted.
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Proposed change
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===============
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Shelved instances can be in one of two possible states: shelved and
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shelved_offloaded (ignoring transitions during shelving and unshelving).
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When in shelved the instance is still on a host but inactive. When in
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shelved_offloaded the instance has been removed from the host and the
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resources it was using there are released.
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Volume operations on an instance in the shelved state are similar to
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any other state when on the host. The operations can be enabled by allowing
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them at the compute API for this state. The existing compute manager code
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does handle this case already; it is merely disabled in the API.
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The shelved_offloaded state is different. In this case the instance is not
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on any host, so functions to attach and detach need to be implemented in
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the API in the same way that the code to detach volumes for deletion is done.
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These will only perform the steps to manage the block device mappings and
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register with cinder. Any actual attachment to a host will be completed
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when the instance is unshelved as usual.
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The compute api attach volume code makes an rpc call to the hosting compute
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manager to select a name for the device, which includes a call into the virt
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driver. This can not be done when the instance is offloaded
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because it is not on a host.
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In fact, devices names are set when an instance is booted
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and there is no guarantee that a name provided by the user will be
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respected. So the new attach method for the shelved_offloaded state will
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defer name selection until the instance is unshelved. This avoids the need
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to call a compute manager at all.
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Alternatives
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------------
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The only clear alternative is to not allow volumes to be attached or detached
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when an instance is shelved.
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Data model impact
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-----------------
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None.
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REST API impact
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---------------
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The attach, detach and swap operations will
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be allowed when the instance is in the shelved and shelved_offloaded states.
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Instead of returning the existing HTTP error 409 (Conflict)
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the return values will be the same as they are for other valid states.
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This change will require an API microversion increment.
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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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None.
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Performance Impact
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------------------
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None.
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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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None.
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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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pmurray
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Other contributors:
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andrea-rosa-m
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Work Items
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----------
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The following changes will be required:
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#. Change the guards on the attach, detach and swap functions in the compute
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API to allow them when the instance is in the shelved state.
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#. Add functions to attach, detach and swap volumes that are be executed
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locally at the API when the instance is in the shelved offloaded state.
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#. Add code to handle device names on unshelve (devices attached in
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shelved_offloaded will have had name selection deferred to unshelve).
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#. Change the guards on the attach, detach and swap functions to allow them
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when the instance is in the shelved_offloaded state.
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Dependencies
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============
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This spec is a step towards allowing boot volumes to be attached and
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detached when in the shelved_offloaded state (see [1]). But this spec
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also provides useful functionality on its own.
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This spec adds more opportunity to get race conditions due to
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conflicting parallel operations, it is important to note that those races
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are not introduced by this change but already exist in nova and they are
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going to be addressed by a different change, please see [2] for more
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information.
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Testing
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=======
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Most of the attach and detach functionality can be tested with unit tests.
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In particular the shelved state is the same as shutdown or stopped.
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New unit tests will be needed for the new attach and detach functions in the
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shelved offloaded state.
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A tempest test will be added to check that the sequence of shelving,
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detaching/attaching volumes and then unshelving leads to a running
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instance with the expected volumes correctly attached.
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Documentation Impact
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====================
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This spec will affect cloud users. They will now be able to perform volume
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operations on shelved instances.
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References
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==========
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[1] https://blueprints.launchpad.net/openstack/?searchtext=detach-boot-volume
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[2] https://review.openstack.org/216578
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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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* - Mitaka
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- Introduced
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