When the 'nova-manage cellv2 discover_hosts' command is run in parallel
during a deployment, it results in simultaneous attempts to map the
same compute or service hosts at the same time, resulting in
tracebacks:
"DBDuplicateEntry: (pymysql.err.IntegrityError) (1062, u\"Duplicate
entry 'compute-0.localdomain' for key 'uniq_host_mappings0host'\")
[SQL: u'INSERT INTO host_mappings (created_at, updated_at, cell_id,
host) VALUES (%(created_at)s, %(updated_at)s, %(cell_id)s,
%(host)s)'] [parameters: {'host': u'compute-0.localdomain',
%'cell_id': 5, 'created_at': datetime.datetime(2019, 4, 10, 15, 20,
%50, 527925), 'updated_at': None}]
This adds more information to the command help and adds a warning
message when duplicate host mappings are detected with guidance about
how to run the command. The command will return 2 if a duplicate host
mapping is encountered and the documentation is updated to explain
this.
This also adds a warning to the scheduler periodic task to recommend
enabling the periodic on only one scheduler to prevent collisions.
We choose to warn and stop instead of ignoring DBDuplicateEntry because
there could potentially be a large number of parallel tasks competing
to insert duplicate records where only one can succeed. If we ignore
and continue to the next record, the large number of tasks will
repeatedly collide in a tight loop until all get through the entire
list of compute hosts that are being mapped. So we instead stop the
colliding task and emit a message.
Closes-Bug: #1824445
Change-Id: Ia7718ce099294e94309103feb9cc2397ff8f5188
5c544c7e2a