3420b77f62
While collecting information because of a question I received about soft delete and shadow tables I realized that the documentation contains bits and pieces here and there, but I couldn't find more. This change summarizes what I found from docs and asking around. I hope you find it useful. Change-Id: I5ff90224cc27c57dc463604559d25298ed7b3f98
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=============================
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Soft Delete and Shadow Tables
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=============================
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Nova has two unrelated features which are called ``soft delete``:
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Soft delete instances that can be restored
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After an instance delete request, the actual delete is
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delayed by a configurable amount of time (config option
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:oslo.config:option:`reclaim_instance_interval`). During the delay,
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the instance is marked to be in state ``SOFT_DELETED`` and can be
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restored (:command:`openstack server restore`) by an admin in order to
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gracefully handle human mistakes. If the instance is not restored during
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the configured delay, a periodic job actually deletes the instance.
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This feature is optional and by default off.
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See also:
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- "Delete, Restore" in `API Guide: Server Concepts
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<https://docs.openstack.org/api-guide/compute/server_concepts.html#server-actions>`_
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- config reference: :oslo.config:option:`reclaim_instance_interval`
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Soft delete database rows to shadow tables
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------------------------------------------
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At an actual instance delete, no DB record is deleted. Instead the
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records are marked as deleted (for example ``instances.deleted``
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in Nova cell databases). This preserves historic information
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for debugging and audit uses. But it also leads to accumulation
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of data in Nova cell DB tables, which may have an effect on
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Nova DB performance as documented in `DB prune deleted rows
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<https://docs.openstack.org/nova/latest/admin/upgrades.html#concepts>`_.
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The records marked as deleted can be cleaned up in multiple stages.
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First you can move them to so-called shadow tables (tables with prefix
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``shadow_`` in Nova cell databases). This is called *archiving the
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deleted rows*. Nova does not query shadow tables, therefore data moved
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to the shadow tables no longer affect DB performance. However storage
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space is still consumed. Then you can actually delete the information
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from the shadow tables. This is called *DB purge*.
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These operations can be performed by nova-manage:
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- https://docs.openstack.org/nova/latest/cli/nova-manage.html#db-archive-deleted-rows
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- https://docs.openstack.org/nova/latest/cli/nova-manage.html#db-purge
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This feature is not optional. Every long-running deployment should
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regularly archive and purge the deleted rows. For example via a cron
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job to regularly call :program:`nova-manage db archive_deleted_rows` and
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:program:`nova-manage db purge`. The tradeoffs between data retention,
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DB performance and storage needs should be considered.
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In the Mitaka release there was an agreement between Nova developers that
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it's not desirable to provide shadow tables for every table in the Nova
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database, `documented in a spec
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<https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/no-more-soft-delete.html>`_.
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Therefore not all information about an instance is preserved in the shadow
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tables. Since then new shadow tables are not introduced.
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