Power Synchronization¶
Baremetal Power Sync¶
Each Baremetal conductor process runs a periodic task which synchronizes the
power state of the nodes between its database and the actual hardware. If the
value of the conductor.force_power_state_during_sync
option is set to true the power state in the database will be forced on
the hardware and if it is set to false the hardware state will be forced
on the database. If this periodic task is enabled, it runs at an interval
defined by the conductor.sync_power_state_interval
config option for those nodes which are not in maintenance. The requests sent
to Baseboard Management Controllers (BMCs) are done with a parallelism
controlled by conductor.sync_power_state_workers.
The motivation to send out requests to BMCs in parallel is to handle
misbehaving BMCs which may delay or even block the synchronization otherwise.
Note
In deployments with many nodes and IPMI as the configured BMC protocol, the default values of a 60 seconds power sync interval and 8 worker threads may lead to a high rate of required retries due to client-side UDP packet loss (visible via the corresponding warnings in the conductor logs). While Ironic automatically retries to get the power status for the affected nodes, the failure rate may be reduced by increasing the power sync cycle, e.g. to 300 seconds, and/or by reducing the number of power sync workers, e.g. to 2. Pleae keep in mind, however, that depending on the concrete setup increasing the power sync interval may have an impact on other components relying on up-to-date power states.
Compute-Baremetal Power Sync¶
Each nova-compute process in the Compute service runs a periodic task which
synchronizes the power state of servers between its database and the compute
driver. If enabled, it runs at an interval defined by the
sync_power_state_interval config option on the nova-compute process.
In case of the compute driver being baremetal driver, this sync will happen
between the databases of the compute and baremetal services. Since the sync
happens on the nova-compute process, the state in the compute database
will be forced on the baremetal database in case of inconsistencies. Hence a
node which was put down using the compute service API cannot be brought up
through the baremetal service API since the power sync task will regard the
compute service’s knowledge of the power state as the source of truth. In order
to get around this disadvantage of the compute-baremetal power sync,
baremetal service does power state change callbacks to the compute service
using external events.
Power State Change Callbacks to the Compute Service¶
Whenever the Baremetal service changes the power state of a node, it can issue a notification to the Compute service. The Compute service will consume this notification and update the power state of the instance in its database. By conveying all the power state changes to the compute service, the baremetal service becomes the source of truth thus preventing the compute service from forcing wrong power states on the physical instance during the compute-baremetal power sync. It also adds the possibility of bringing up/down a physical instance through the baremetal service API even if it was put down/up through the compute service API.
This change requires the nova section and the necessary
authentication options like the nova.auth_url to be
defined in the configuration file of the baremetal service. If it is not
configured the baremetal service will not be able to send notifications to the
compute service and it will fall back to the behaviour of the compute service
forcing power states on the baremetal service during the power sync.
See nova group for more details on the available config
options.
In case of baremetal stand alone deployments where there is no compute service
running, the nova.send_power_notifications config option
should be set to False to disable power state change callbacks to the
compute service.
Note
The baremetal service sends notifications to the compute service only if
the target power state is power on or power off. Other error
and None states will be ignored. In situations where the power state
change is originally coming from the compute service, the notification
will still be sent by the baremetal service and it will be a no-op on the
compute service side with a debug log stating the node is already powering
on/off.
Note
Although an exclusive lock is used when sending notifications to the compute service, there can still be a race condition if the compute-baremetal power sync happens to happen a nano-second before the power state change event is received from the baremetal service in which case the power state from compute service’s database will be forced on the node.
