Soft updates: a technique for eliminating most synchronous writes in the fast filesystem

Abstract

Traditionally, filesystem consistency has been maintained across system failures either by using synchronous writes to sequence dependent metadata updates or by using write-ahead logging to atomically group them. Soft updates, an alternative to these approaches, is an implementation mechanism that tracks and enforces metadata update dependencies to ensure that the disk image is always kept consistent. The use of soft updates obviates the need for a separate log or for most synchronous writes. Indeed, the ability of soft updates to aggregate many operations previously done individually and synchronously reduces the number of disk writes by 40 to 70% for fileintensive environments (e.g., program development, mail servers, etc.). In addition to performance enhancement, soft updates can also maintain better disk consistency. By ensuring that the only inconsistencies are unclaimed blocks or inodes, soft updates can eliminate the need to run a filesystem check program after every s...