A trace-driven analysis of the Unix 4.2 BSD file system

We analyzed the UNIX 4.2 BSD file system by recording user-level activity in trace files and writing programs to analyze the traces. The tracer did not record individual read and write operations, yet still provided tight bounds on what information was accessed and when. The trace analysis shows that the aver-age file system bandwidth needed per user is low (a few hun-dred bytes per second). Most of the files accessed are open only a short time and are accessed sequentially. Most new informa-tion is deleted or overwritten within a few minutes of its crea-tion. We also wrote a simulator that uses the traces to predict the performance of caches for disk blocks. The moderate-sized caches used in UNIX reduce disk traffic for file blocks by about 50%, but larger caches (several megabytes) can eliminate 90% or more of all disk traffic. With those large caches, large block sizes (16 kbytes or more) result in the fewest disk accesses. Trace-Driven Analysis of 4.2 BSD File System January 2, 1993 1