About system capacity and service group load

The Load and Capacity construct allows the administrator to define a fixed amount of resources a server provides (Capacity), and a fixed amount of resources a specific service group is expected to utilize (Load).

The system attribute Capacity sets a fixed load-handling capacity for servers. Define this attribute based on system requirements.

The service group attribute Load sets a fixed demand for service groups. Define this attribute based on application requirements.

When a service group is brought online, its load is subtracted from the system's capacity to determine available capacity. VCS maintains this info in the attribute AvailableCapacity.

When a failover occurs, VCS determines which system has the highest available capacity and starts the service group on that system. During a failover involving multiple service groups, VCS makes failover decisions serially to facilitate a proper load-based choice.

System capacity is a soft restriction; in some situations, value of the Capacity attribute could be less than zero. During some operations, including cascading failures, the value of the AvailableCapacity attribute could be negative.

Static load versus dynamic load

Dynamic load is an integral component of the Service Group Workload Management framework. Typically, HAD sets remaining capacity with the function:

AvailableCapacity = Capacity - (sum of Load values of all online service groups)

If the DynamicLoad attribute is defined, its value overrides the calculated Load values with the function:

AvailableCapacity = Capacity - DynamicLoad

This enables better control of system loading values than estimated service group loading (static load). However, this requires setting up and maintaining a load estimation package outside VCS. It also requires modifying the configuration file main.cf manually.

Note that the DynamicLoad (specified with hasys -load) is subtracted from the Capacity as an integer and not a percentage value. For example, if a system's capacity is 200 and the load estimation package determines the server is 80 percent loaded, it must inform VCS that the DynamicLoad value is 160 (not 80).

About overload warning

Overload warning provides the notification component of the Load policy. When a server sustains the preset load level (set by the attribute LoadWarningLevel) for a preset time (set by the attribute LoadTimeThreshold), VCS invokes the loadwarning trigger.

See Using event triggers

See System attributes.

The loadwarning trigger is a user-defined script or application designed to carry out specific actions. It is invoked once, when system load exceeds the LoadWarningLevel for the LoadTimeThreshold. It is not invoked again until the LoadTimeCounter, which determines how many seconds system load has been above LoadWarningLevel, is reset.