The Optimal Allocation Policy of IBM 3330 Single Density Disk Devices.
Frederick Carl Mayer · 1978
As a data processing department grows and Its use of direct access storage devices Increases, It becomes necessary to efficiently allocate and manage secondary storage. On the IBM 370 computer, tvo allocation techniques are usually employed. These are cylinder and track allocation. As data acts (flics) are created and deleted, some degree of pack fragmentation Is unavoidable. The problem with pack fragmentation Is that It can become so severe that data set allocation is Impossible. One possible solution is to reorganize (compress) the pack; i.e. move data sets so as to increase the amount of contiguous free spaces. Most reorganiratlon requires exclusive control of the computer. Depending on the frequency of this reorganization, it can be very time consuming and more importantly, It can be very costly. A simulation model is built to examine the tvo above mentioned techniques. The main emphasis Is on the comparison of cylinder allocation and track allocation. The model uses GASP II to simulate an IBM 370/165 with 3330 MOD 1 type disk drives. Statistics are collected from the Systems Management Facility (SMF) of the OS/MVT operating system. Every tine a flic, which van opened for output, la closed a type 15 record la urltten by SMF. Thlo record Includes such information as flic creation time, sire, and data act charactcrlstlea. With the collection of thcoc records It In possible to determine needed simulation statistics. Inter arrival tines and average file size can be calculated. There are also type 17 SMF records written every tine a flic Is scratched. The scratch records, In combination with the type 15 records, provide the average life of a flic. These three statistics (arrival time, size, and flic life) are needed to build the disk allocation model. Several considerations were designed Into the model. While most computer Installations with disk secondary storage devices have several modules on line at one time, this simulation model will examine only one IBM 3330 MOD 1 pack. Also, statistics were calculated with SMF records collected betveen the times of 08:00 and 16:00. If not restricted to one pack and this tine period, the model would have become too large and unmanagablo. The operating system to be simulated is 0S/MVT 21.8A and the allocation algorithm uses the fix fit technique. All files are allocated as contiguous, one extent, physical sequential data sets. The results of the Investigation show that cylinder allocation techniques yield less pack fragmentation. This type of allocation does, however, have a greater potential for wasting space on the disk pack.