Vijay Chidambaram — Page 2

arXiv2018

Finding Crash-Consistency Bugs with Bounded Black-Box Crash Testing

We present a new approach to testing file-system crash consistency: bounded black-box crash testing (B3). B3 tests the file system in a black-box manner using workloads of file-system operations. Since the space of possible workloads is infinite, B3 bounds this space based on parameters such as the number of file-system operations or which operations to include, and exhaustively generates workloads within this bounded space. Each workload is tested on the target file system by simulating power-loss crashes while the workload is being executed, and checking if the file system recovers to a correct state after each crash. B3 builds upon insights derived from our study of crash-consistency bugs reported in Linux file systems in the last five years. We observed that most reported bugs can be reproduced using small workloads of three or fewer file-system operations on a newly-created file system, and that all reported bugs result from crashes after fsync() related system calls. We build two tools, CrashMonkey and ACE, to demonstrate the effectiveness of this approach. Our tools are able to find 24 out of the 26 crash-consistency bugs reported in the last five years. Our tools also revealed 10 new crash-consistency bugs in widely-used, mature Linux file systems, seven of which existed in the kernel since 2014. Our tools also found a crash-consistency bug in a verified file system, FSCQ. The new bugs result in severe consequences like broken rename atomicity and loss of persisted files.

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arXiv2017

Analyzing IO Amplification in Linux File Systems

We present the first systematic analysis of read, write, and space amplification in Linux file systems. While many researchers are tackling write amplification in key-value stores, IO amplification in file systems has been largely unexplored. We analyze data and metadata operations on five widely-used Linux file systems: ext2, ext4, XFS, btrfs, and F2FS. We find that data operations result in significant write amplification (2-32X) and that metadata operations have a large IO cost. For example, a single rename requires 648 KB write IO in btrfs. We also find that small random reads result in read amplification of 2-13X. Based on these observations, we present the CReWS conjecture about the relationship between IO amplification, consistency, and storage space utilization. We hope this paper spurs people to design future file systems with less IO amplification, especially for non-volatile memory technologies.

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