The author presents a new approach to modeling file systems using queueing networks. The delays due to locking the files are modeled using service centers whose service times and probabilities of access are estimated from the values of measurable quantities. The model of a lock is based on the analysis of execution of transactions in the system. The lock for every file is modeled as a sequence of service centers. The decomposition method can be used to solve the model, which allows multiple classes of transactions and shared files to be represented. An example involving measurement data collected in a small business installation is given to compare performance measures provided by the simulation and analytic models.