What is a journaling file system and why is it important?

Journaling file systems maintain a log (journal) of changes before writing to main file system. On crash, system replays/reverts journal entries to reach consistent state - much faster than full fsck. Types: Write-ahead logging (journal metadata only or full data), Log-structured (entire FS is log). Journaling adds write overhead but ensures consistency. Common journaling file systems: ext4, NTFS, XFS, ZFS. ZFS uses copy-on-write instead of traditional journaling.