Soft updates: A solution to the metadata update problem in file systems

Metadata updates, such as file creation and block allocation, have consiste ntly been identified as a source of performance, integrity, security, and a vailability problems for file systems. Soft updates is an implementation te chnique for low-cost sequencing of fine-grained updates to write-back cache blocks. Using soft updates to track and enforce metadata update dependenci es, a file system can safely use delayed writes for almost all file operati ons. This article describes soft updates, their incorporation into the 4.4B SD fast file system, and the resulting effects on the system. We show that a disk-based file system using soft updates achieves memory-based file syst em performance while providing stronger integrity and security guarantees t han most disk-based file systems. For workloads that frequently perform upd ates on metadata (such as creating and deleting files), this improves perfo rmance 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 t o an aggressive write-ahead logging approach. In addition, soft updates can improve file system availability by relegating crash-recovery assistance ( e.g., the fsck utility) to an optional and background role, reducing file s ystem recovery time to less than one second.