Power Loss

Overview

Power distribution in a computer system is complex. It typically flows from a wall outlet to power supplies to voltage regulators to system components. Other devices may exist between the wall outlet and the power supplies, such as an Uninterruptible Power Supply (UPS) or an Enclosure Power Distribution Unit (ePDU).

A brownout is a partial reduction in voltage to the power supplies. In some situations, the standby voltage rails are still powered on but the main voltage rails are powered off. As a result, the BMC may still be running, but the host processor and other system components have lost power.

A blackout is a complete loss of power to the power supplies.

In a multiple chassis system, a brownout or blackout might only occur in some of the chassis.

System behavior during a brownout

If the chassis was powered off when the brownout occurred, the phosphor-power-sequencer application will take no action.

If the chassis was powered on when the brownout occurred, the power sequencer device will normally change the chassis power good (pgood) signal from true to false. phosphor-power-sequencer will isolate the failure to the power supply rail. phosphor-power-sequencer will log a power supply error. This error will specify the problem was the input voltage rather than the power supply hardware. See Power Good Faults for more information.

System behavior during a blackout

Single chassis system

The system loses all power. It will be completely off until utility power is restored.

When power is restored, if the system was previously powered on, it may be automatically powered on again by the phosphor-chassis-state-manager application. This depends on the Auto Power Restart settings.

Multiple chassis system

If the blackout affects all chassis, the system loses all power. It will behave as described above for a single chassis system.

If the blackout only affects some chassis, the following steps will occur:

  • The phosphor-chassis-power application will do the following:
    • Detect which chassis are experiencing a blackout.
    • Set the Status property of the xyz.openbmc_project.State.Decorator.PowerSystemInputs D-Bus interface to Fault for the chassis experiencing a blackout.
  • The Available property will be set to false for chassis that are experiencing a blackout.
    • This is due to the PowerSystemInputs Status property being set to Fault for the chassis.
  • If the BMC was reset by hardware due to the blackout, the following will occur:
    • phosphor-power-sequencer will read the pgood signal for each chassis and use that value for the state and pgood properties of the org.openbmc.control.Power D-Bus interface for the chassis.
      • For chassis experiencing a blackout, the state and pgood properties will be set to 0.
    • phosphor-chassis-state-manager will obtain the previous on/off state of each chassis from saved data.
    • phosphor-chassis-state-manager will determine if a power problem has occurred by checking if the following are all true:
      • At least one chassis was previously on and is now off.
      • The host operating system is not running or communicating
    • If a power problem has occurred, phosphor-chassis-state-manager will do the following:
      • Log an error
      • Power the system off and then on again. The chassis with Available set to false will not be powered on.

See Chassis Status for more information on the D-Bus chassis status properties.