Merge "Update cheesecake promotion specification"
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..
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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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Promote Secondary Backend After Failover (Cheesecake)
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=====================================================
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https://blueprints.launchpad.net/cinder/+spec/replication-backend-promotion
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Problem description
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===================
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After failing backend A over to backend B, there is not a mechanism in
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Cinder to promote backend B to the master backend in order to then replicate
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to a backend C.
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Current Workflow
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----------------
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1. Setup Replication
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2. Failure Occurs
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3. Fail over
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4. Promote Secondary Backend
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a. Freeze backend to prevent manage operations
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b. Stop cinder volume service
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c. Update cinder.conf to have backend A replaced with B and B with C
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d. *Hack db to set backend to no longer be in 'failed-over' state*
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* This is the step this spec is concerned with
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* Example:
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::
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update services set disabled=0,
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disabled_reason=NULL,
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replication_status='enabled',
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active_backend_id=NULL
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where id=3;
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e. Start cinder volume service
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f. Unfreeze backend
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Use Cases
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=========
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There was a fire in my data center and my primary backend (A) was destroyed.
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Luckily, I was replicating that backend to backend (B). After failing over
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to backend B and repairing the data center, we installed backend C to be a
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new replication target for B.
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Proposed change
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===============
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Add the following commands to reset the active backend for a host.
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::
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cinder reset-active-backend <backend-name>
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PUT /os-services/reset_active_backend {"host": <backend-name>}
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Equivalent to:
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::
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update services set disabled=0,
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disabled_reason=NULL,
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replication_status='disabled',
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active_backend_id=NULL
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where host='<backend-name>;
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The following to re-enable replication from backend B -> C.
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::
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cinder replication-enable <backend-name>
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PUT /os-services/replication_enable {"host": <backend-name>}
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Equivalent to:
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::
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update services set replication_status='enabled'
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where host='<backend-name>;
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Alternatives
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------------
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Instead of 'admin' APIs, we add cinder-manage commands.
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::
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cinder-manage reset-active-backend <backend-host>
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cinder-manage replication-enable <backend-name>
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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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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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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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?
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Work Items
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----------
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* Implement cinder reset-active-backend API
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* Implement cinder replication-enable API
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* Document post-fail-over recovery in Admin guide
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Dependencies
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============
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None
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Testing
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=======
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None
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Documentation Impact
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====================
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Documentation in the Admin guide for how to perform a backend promotion.
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References
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==========
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None
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235
specs/queens/cheesecake-promote-backend.rst
Normal file
235
specs/queens/cheesecake-promote-backend.rst
Normal file
@ -0,0 +1,235 @@
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..
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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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Promote Replication Target (Cheesecake)
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=====================================================
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https://blueprints.launchpad.net/cinder/+spec/replication-backend-promotion
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Problem description
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===================
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After failing backend A over to backend B, there is not a mechanism in
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Cinder to promote backend B to the master backend in order to then replicate
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to a backend C. We also lack some management commands to help rebuild after a
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disaster if states get out of sync.
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Current Workflow
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----------------
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1. Setup Replication
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2. Failure Occurs
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3. Fail over
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4. Promote Secondary Backend
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a. Freeze backend to prevent manage operations
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b. Stop cinder volume service
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c. Update cinder.conf to have backend A replaced with B and B with C
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d. *Hack db to set backend to no longer be in 'failed-over' state*
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* This is the step this spec is concerned with
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* Example:
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::
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update services set disabled=0,
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disabled_reason=NULL,
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replication_status='enabled',
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active_backend_id=NULL
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where id=3;
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e. Start cinder volume service
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f. Unfreeze backend
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Use Cases
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=========
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There was a fire in my data center and my primary backend (A) was destroyed.
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Luckily, I was replicating that backend to backend (B). After failing over
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to backend B and repairing the data center, we installed backend C to be a
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new replication target for B.
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There is also a case where, for whatever reason, the replication state gets out
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of sync with reality. In a situation where the status and active backend id
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need to be adjusted manually by the cloud admin while recovering from a
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disaster.
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Proposed change
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===============
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To handle the case where the Cinder DB just needs to be synchronized with the
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real world we will add the following cinder manage commands to reset the active
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backend for a host. Similar to reset-state for volumes this will just do DB
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operations. The assumption being that the cinder.conf has already been updated,
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and the volume service is stopped, in a disabled state, and probably frozen.
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The manage command will verify that the service is stopped, disabled, and
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frozen.
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::
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cinder-manage reset-active-backend replication_status=<status> \
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<active_backend_id> <backend-host>
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Equivalent to:
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::
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update services set replication_status='<status>',
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active_backend_id='<id>',
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where host='<backend-name>';
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Where the defaults for `status` will be disabled and the `active_backend_id`
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will be None. The target for this could also be a cluster in A-A deployments.
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Note: It will be up to the Admin to re-enabled the service.
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That gives us the ability to avoid having an admin go manually run DB commands,
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but does *NOT* allow for doing this in an "online" recovery. The volume service
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must be offline for this to work safely.
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Making this change will require drivers to make adjustments to their
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replication states upon initialization. As-is we don't really have much
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definition around what is allowed to change in cinder.conf and how much the
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driver should support for changes to replication targets and things like that.
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There is sort of an implied contract that when `__init__` or `do_setup` is
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called the driver *should* make their current replication state match what they
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are given in the config. This is a little bit problematic today though as there
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isn't a mechanism for them to update DB entries like
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`replication_extended_status` or `replication_driver_data`. To solve that gap,
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and allow for this new officially supported method of modifying replication
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status when offline, we will introduce a new driver api method
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::
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update_host_replication(replication_status, active_backend_id, volumes, groups)
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This method will be called immediately following `do_setup` and will return a
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model update for the service as well as a list of updates for the volumes and
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groups if the driver supports replication groups. The `group` parameter will
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default to None if not defined.
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The drivers will need to be able to take appropriate steps to get the system
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into the desired state based on the current DB state (which was theoretically
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modifed before startup by the new cinder-manage command) and the current
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cinder.conf.
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If not implemented things will continue to work as they do today, and require
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that the admin potentially take more drastic measures to recover after
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performing a failover. The goal here is to not break any existing
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functionality, but to add enough infrastructure for drivers to behave better.
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When we do this we might require some way of fencing to prevent multiple A-A
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driver instances from doing the setup at the same time as it will more than
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likely be problematic for some backends, and could risk data loss if more than
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one is modifying replication states at the same time. As a simple solution we
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could use a new replication_status like `updating` for the service, and only
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allow them to call setup_replication if the status is not set to that. This
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status can also be beneficial for an admin to know what is happening if the
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process of updating via the driver takes noticeable amounts of time.
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Doing it this way should also allow for later on making "online" updates where
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we can utilize that same driver hook to modify replication states. This spec
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and initial implementation does not aim to cover that scenario.
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Alternatives
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------------
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We could add admin API's to Cinder. Those API's could do the DB updates and
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ping the drivers. The downside is that it requires the API and volume service
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to be online, which may be problematic in the scenario that you are picking up
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pieces after a disaster.
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Later on we can look into doing "online" promotions where the volume service
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does not need to be offline. Similar code in the drivers would be required, but
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the complexity gets increased rapidly by trying to support this.
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There was also discussion about using new admin API's which would modify a db
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state that tracks replication info. The downside to this is that we will move
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into a scenario where the running config and state doesn't match the configured
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state.
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Following on that path of tracking replication state in the db, we could go to
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the extreme and move all of the replication configuration to be done via API's.
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We can then track state, and provide drivers with diffs as the state changes.
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In the longer term that addresses the runtime vs config state disparity, but it
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will be a significant change in workflow and deployments. Not to mention would
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require somewhat major changes to drivers implementing replication.
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Data model impact
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-----------------
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A new status for the service/cluster will be added.
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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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None
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Performance Impact
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------------------
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Volume service startup will probably take a performance hit, depending on the
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backend and how many replicated volumes need to be modified and updated.
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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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Driver maintainers will need to potentially implement this new functionality,
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and be aware of the implications of how/when replication configuration and
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status can be adjusted.
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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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jbernard
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Work Items
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----------
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* Implement cinder-manage reset-active-backend command
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* Implement volume manager changes to allow for `update_host_replication` to be
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used at startup by drivers.
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* Open a bug against each backend that supports replication and needs an
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update as a result of this change.
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Dependencies
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============
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None
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Testing
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||||
=======
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None
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||||
Documentation Impact
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||||
====================
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||||
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Documentation in the Admin guide for how to perform a backend promotion, and
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updating the devref for driver developers to explain the expectations of
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drivers implementing replication.
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References
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||||
==========
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||||
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||||
None
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||||
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