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The ADvanced Systems Laboratory (ADSL)
Publication abstract

ffsck: The Fast File System Checker

Ao Ma*, Chris Dragga, Andrea C. Arpaci-Dusseau, Remzi H. Arpaci-Dusseau
Department of Computer Sciences, University of Wisconsin-Madison

Marshall Kirk McKusick

*Also affiliated with Backup Recovery Systems Division, EMC Corporation


Crash failures, hardware errors, and file system bugs can corrupt file systems and cause data loss, despite the presence of journals and similar preventive techniques. While consistency checkers such as fsck can detect this corruption and restore a damaged image to a usable state, they are generally created as an afterthought, to be run only at rare intervals. Thus, checkers operate slowly, causing significant downtime for large scale storage systems when they are needed.

We address this dilemma by treating the checker as a key component of the overall file system (and not merely a peripheral add-on). To this end, we present a modified ext3 file system, rext3, to directly support the fast file system checker, ffsck. The rext3 file system colocates and self-identifies its metadata blocks, removing the need for costly seeks and tree traversals during checking. These modifications to the file system allow ffsck to scan and repair the file system at rates approaching the full sequential bandwidth of the underlying device. In addition, we demonstrate that rext3 performs competitively with ext3 in most cases and exceeds it in handling random reads and large writes. Finally, we apply our principles to FFS, the default FreeBSD file system, and its checker, doing so in a lightweight fashion that preserves the file-system layout while still providing some of the gains in performance from ffsck.

Full Paper: PDF, BibTex