Placement uses alembic to manage the DB version for schema changes.
However, changes with data manupulation should be separated from the
schema changes since the table can be locked and in the worst case
it breaks the service for backward incompatible changes.
We could handle them as a task that is done in a service down time.
However, to minimize the down time, it is better to have the concepts
of online data migration which has been a traditional way to handle
those data manipulation changes in nova.
This patch adds online data migration command to placement to enable
operators to manipulate DB data while the service is running:
placement-manage db online_data_migrations [--max-count]
where --max-count controls the maximum number of objects to migrate
in a given call. If not specified, migration will occur in batches
of 50 until fully complete.
Change-Id: I9cef6829513d9a54d110426baf6bcc312554e3e7
80fa50187a