In this paper we look in detail at a computer system consisting of one Central Processing Unit (CP) and one Disk. It is visited by jobs with a special kind of workload, one should think for example of database queries. During a visit to the computer system a job is served several times at the CP and several times at the Disk.
We give a queueing model representing this system. This model is used to estimate the mean and variance of the response time of these jobs. Numerical examples are compared with a simulation of the system to check the correctness of this method.