[X4U] Disk Fragmentation item from Apple Support

[Ed Gould]

http://support.apple.com/kb/HT1375
Do I need to optimize?

You probably won't need to optimize at all if you use Mac OS X.  
Here's why:

Hard disk capacity is generally much greater now than a few years  
ago. With more free space available, the file system doesn't need to  
fill up every "nook and cranny." Mac OS Extended formatting (HFS  
Plus) avoids reusing space from deleted files as much as possible, to  
avoid prematurely filling small areas of recently-freed space.
Mac OS X 10.2 and later includes delayed allocation for Mac OS X  
Extended-formatted volumes. This allows a number of small allocations  
to be combined into a single large allocation in one area of the disk.
Fragmentation was often caused by continually appending data to  
existing files, especially with resource forks. With faster hard  
drives and better caching, as well as the new application packaging  
format, many applications simply rewrite the entire file each time.  
Mac OS X 10.3 Panther can also automatically defragment such slow- 
growing files. This process is sometimes known as "Hot-File-Adaptive- 
Clustering."
Aggressive read-ahead and write-behind caching means that minor  
fragmentation has less effect on perceived system performance.

For these reasons, there is little benefit to defragmenting.

Note:Mac OS X systems use hundreds of thousands of small files, many  
of which are rarely accessed. Optimizing them can be a major effort  
for very little practical gain. There is also a chance that one of  
the files placed in the "hot band" for rapid reads during system  
startup might be moved during defragmentation, which would decrease  
performance.

If you think you might need to defragment

Try restarting first. It might help, and it's easy to do.

If your disks are almost full, and you often modify or create large  
files (such as editing video, but see the Tip below if you use iMovie  
and Mac OS X 10.3), there's a chance the disks could be fragmented.  
In this case, you might benefit from defragmentation, which can be  
performed with some third-party disk utilities.

Another option is to back up your important files, erase the hard  
disk, then reinstall Mac OS X and your backed up files.

Tip: If you use iMovie with Mac OS X 10.3 Panther, and FileVault is  
enabled, performance issues can occur if your project is located in  
your encrypted home folder (including the desktop). See iMovie: Using  
FileVault Can Affect Performance.