ffsck: The Fast File System Checker

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 co-locates 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 andrepair 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.