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Before describing the system model, we define
the terms behavior, goals and actions. In Polus,
behavior is a defined as a set of QoS dimensions such
as .
These dimensions are also referred to as observables. Goals are defined as
threshold values on behavior dimensions. e.g.
response-time for reads should less than 5 sec, a client's average throughput
should be 150 MBps etc. The definition of actions is
domain-specific. In Polus, the actions are divided
into two categories: a) tunable parameters i.e., changing the value of
configuration parameters such a prefetch size,
clean-delay, etc., and b) internal actions such as migration, replication, and
back-up of data. The model of the system is shown in figure 4. It
consists of two key entities: the management software and the managed system.
The management software is assigned QoS goals and is
responsible for ensuring that the managed system meets these goals. The
interaction between the management software and the managed system is via sensors
and actuators. Monitors gather information about the managed system,
while the actuators effect the actions invoked by the management software on
the managed system. The managed system consists of physical components
that interact to service the application requests. The components service the
requests in a particular sequence, with each component processing the
information before handing it to the next component. For example, within a SAN
file system, the components are client machines, metadata servers, storage
controllers and disks. A component has pre-defined properties such as the
maximum number of requests that it can service, the error rate, the average
down-time etc. Each component consists of one of more atomic entities referred
to as resources. In other words, resources serve as an abstraction to
refer to components in a generic fashion. In Polus,
the possible resources are memory, CPU, network, and storage. For example, the
storage controller component consists of memory and storage resources,
the client machine component consists of CPU, memory and network resource. Sensors collects information about the state of the
managed system. The state of the system is defined as a quadruple
, defined as follows:
Actuators invoke the actions selected
by the management software. The impact of invoking an action is not constant
and is a function of the .
Furthermore, whether an action can be invoked or not is dependent on the
(i.e. not all actions are applicable within a particular
).
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