SDI Seminar

Speaker: Liddy Shriver, Bell Labs

Storage device modeling using a composite device model

Date: August 14, 1997


Our approach to modeling storage devices is to model the individual physical components of the device, such as queues, caches, and disk mechanisms, and then compose the component models. Each component model determines its behavior from the specification of the entering workload and the lower-level device behavior. To support the lower-level component model in determining its behavior, each component model creates a modified workload specification to support the manner that the physical component would modify the entering workload. Modifying the workload specification allows us, for example, to capture the altered spatial locality that occurs when queues reorder their requests.

Our model predicts the device behavior in terms of response time within a relative error ranging from 2% to 30% for interesting subsets of the domain of devices and workloads. To demonstrate this, the model has been validated with synthetic traces of parallel scientific file system workloads and video-on-demand applications and traces of transaction processing applications.

Our contributions to the area of performance modeling for storage devices include the following.

  • An infrastructure for developing a composite model. The infrastructure supports the development of more complicated devices and workloads than we have validated.
  • Methods to approximate the mean seek time and rotational latency of a disk mechanism using measures of workload spatial locality.
  • Methods to approximate the miss probability and the full- and partial-hit probabilities in an I/O system's data caches using measures of workload spatial locality.
  • Methods to approximate the queue delay for non-FCFS scheduling algorithms using a description of the workload arrival process.