<P> A physical journal logs an advance copy of every block that will later be written to the main file system . If there is a crash when the main file system is being written to, the write can simply be replayed to completion when the file system is next mounted . If there is a crash when the write is being logged to the journal, the partial write will have a missing or mismatched checksum and can be ignored at next mount . </P> <P> Physical journals impose a significant performance penalty because every changed block must be committed twice to storage, but may be acceptable when absolute fault protection is required . </P> <P> A logical journal stores only changes to file metadata in the journal, and trades fault tolerance for substantially better write performance . A file system with a logical journal still recovers quickly after a crash, but may allow unjournaled file data and journaled metadata to fall out of sync with each other, causing data corruption . </P> <P> For example, appending to a file may involve three separate writes to: </P>

Difference between journaling and non journaling file system