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ACM Transactions
on Computer Systems, Vol. 18, No. 2, May 2000, Pages 127–153.
Soft Updates:
A Solution to the Metadata
Update Problem in File Systems
Gregory R. Ganger*, Marshall
Kirk McKusick**, Craig A.N. Soules*, Yale N. Patt***
Carnegie Mellon University*
McKusick.com**
University of Texas, Austin***
Abstract
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 system. 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 availability 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.
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