Soft updates

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Metadata updates, such as file creation and block allocation, have consistently been identified as a source of performance, integrity, security, and availability problems for file systems. Soft updates is an implementation technique for low-cost sequencing of fine-grained updates to write-back cache blocks. Using soft updates to track and enforce metadata update dependencies, a file system can safely use delayed writes for almost all file operations. This article describes soft updates, their incorporation into the 4.4BSD fast file system, and the resulting effects on the sytem. We show that a disk-based file system using soft updates achieves memory-based file system performance while providing stronger integrity and security guarantees than most disk-based file systems. For workloads that frequently perform updates on metadata (such as creating and deleting files), this improves performance by more than a factor of two and up to a factor of 20 when compared to the conventional synchronous write approach and by 4-19% when compared to an aggressive write-ahead logging approach. In addition, soft updates can improve file system availablity by relegating crash-recovery assistance (e.g., the fsck utility) to an optional and background role, reducing file system recovery time to less than one second.