Journaled Soft-Updates

Because soft updates prevent most inconsistencies, the journaling need only deal with tracking those inconsistencies that soft updates fails to address. Specifically, the journal contains the information needed to recover the block and inode resources that have been freed but whose freed status failed to make it to disk before a system failure. After a crash, a variant of the venerable fsck program runs through the journal to identify and free the lost resources. Only if a corruption of the log is detected is it necessary to run background fsck. The journal is tiny, 16Mb is usually enough independent of filesystem size. Although journal processing needs to be done before restarting, the processing time is typically just a few seconds and in the worst case a minute. It is not necessary to build a new filesystem to use soft-updates journalling. The addition or deletion of soft-updates journaling to existing FreeBSD fast filesystems is done using the tunefs program.