Soft updates

In computer file systems, soft updates are an approach to maintaining disk integrity after a crash or power outage. They are an alternative to journaling file systems.

Instead of duplicating metadata writes in a journal, soft updates work by properly ordering the metadata writes to guarantee consistency after a crash. Like journaling, soft updates do not guarantee no lost data, but do make sure the filesystem is consistent.

An advantage to soft updates is that filesystems taking advantage of soft updates can be mounted almost instantaneously.

Soft updates were first introduced to FreeBSD by Marshall Kirk McKusick and are now available across the BSD's.

External link

• Information about soft updates, snapshots, and back-ground Fsck (mckusick.com)
• Mckusick, M. and Ganger, G. 1999 Soft Updates: A Technique for Eliminating Most Synchronous Writes in the Fast Filesystem. 18 pg pdf file
• Seltzer, et. al. 2000 Journaling Versus Soft Updates: Asynchronous Meta-data Protection in File Systems. html 15 pg pdf file