Disk File Allocation Based on the Buddy System

Abstract

The buddy system is known for its speed and simplicity. However, high internal and external fragmentation have made it unattractive for use in operating system file layout. A variant of the binary buddy system that reduces fragmentation is described. Files are allocated on up to t extents, and inoptimally allocated files are periodically reallocated. The Dartmouth Time-Sharing System (DTSS) uses this method. Several installations of DTSS, representing different classes of workload, are studied to measure the method's performance. Internal fragmentation varies from 2-6 percent, and external fragmentation varies from 0-10 percent for expected request sizes. Less than 0.1 percent of the CPU is spent executing the algorithm. In addition, most files are stored contiguously on disk. The mean number of extents per file is less than 1.5, and the upper bound is t. Compared to the tile layout method used by UNIX, the buddy system results in more efficient access but less efficient utilization of disk space. As disks become larger and less expensive per byte, strategies that achieve efficient I/O throughput at the expense of some storage loss become increasingly attractive. © 1987, ACM. All rights reserved.