Accurate and efficient replaying of file system traces

Abstract

Replaying traces is a time-honored method for benchmarking, stress-testing, and debugging systems--and more recently--forensic analysis. One benefit to replaying traces is the reproducibility of the exact set of operations that were captured during a specific workload. Existing trace capture and replay systems operate at different levels: network packets, disk device drivers, network file systems, or system calls. System call replayers miss memory-mapped operations and cannot replay I/O-intensive workloads at original speeds. Traces captured at other levels miss vital information that is available only at the file system level.We designed and implemented Replayfs, the first system for replaying file system traces at the VFS level. The VFS is the most appropriate level for replaying file system traces because all operations are reproduced in a manner that is most relevant to file-system developers. Thanks to the uniform VFS API, traces can be replayed transparently onto any existing file system, even a different one than the one originally traced, without modifying existing file systems. Replayfs's user-level compiler prepares a trace to be replayed efficiently in the kernel where multiple kernel threads prefetch and schedule the replay of file system operations precisely and efficiently. These techniques allow us to replay I/O-intensive traces at different speeds, and even accelerate them on the same hardware that the trace was captured on originally.