  The need for high availability (HA) and disaster recovery (DR) in IT
environment is more stringent than most of the other sectors of enterprises.
Many businesses require the availability of business-critical applications 24
hours a day, seven days a week, and can afford no data loss in the event of a
disaster. It is vital that the IT infrastructure is resilient with regard to
disruption, even site failures, and that business operations can continue
without significant impact. As a result, DR has gained great importance in IT.
Clustering of multiple industries standard servers together to allow workload
sharing and fail-over capabilities is a low cost approach. In this paper, we
present the availability model through Semi-Markov Process (SMP) and also
analyze the difference in downtime of the SMP model and the approximate
Continuous Time Markov Chain (CTMC) model. To acquire system availability, we
perform numerical analysis and SHARPE tool evaluation.
