Editing Soft updates (section)

== Method of operation ==

Soft updates allow only asynchronous metadata writes that do not render the on-disk file system inconsistent, or that the only inconsistency that ever happens is a storage space leak (space marked allocated when not used by any file).  It avoids having to do ordered synchronous metadata writes by temporarily "rolling back" any part of a metadata block that depends on another potentially non-flushed or partially rolled-back block when writing it.

In effect, blocks may be flushed at any time and the soft updates code will always provide the disk a consistent version of the metadata (as long as it knows which blocks have physically been flushed). Recovery then simply becomes a matter of running a background walk of the file system when it is next mounted to [[garbage collection (computer science)|garbage collect]] any allocated space that has been orphaned. This also permits the file system to selectively flush certain files without having to flush all metadata blocks or all of the records.

Data that is unlinked from the metadata dependency graph before writing it to disk has begun, does not need to be written to disk at all. For example, creating a file, using it for a short period of time, and then deleting it may cause no disk activity at all.

Soft updates require periodic flushing of the metadata to nonvolatile storage.