Filesystem journaling has nothing to do with defragging. Journaling is a feature that maintains filesystem integrity by keeping a "journal" of activity. If a system crash occurs, the "journal" can be used to detect files that were corrupted when the system crashed.
Defragmenting is a process that ensures that files are stored contiguously on the disk for faster performance.
Defragging -> SPEED
Journaling -> RELIABILITY
OS X has both filesystem journaling and automatic defragmenting.
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