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Apple Mac OS X FAQ
Friday, October 31, 2003
 
Mac OS X 10.3 has automatic file defragmentation.
Mac OS X 10.3 has automatic file defragmentation.

Basically, when a file is accessed, a check is made to see if it is fragmented (split into several different sectors on the Hard Drive), If so, and if it is less than 20 MB in size, the filesystem will copy the file over to a contiguous area on the HD that will hold the file in it's entirety in concurrent sectors, and then free up the HD space the fragmented version used to occupy.

This method is know as "Hot-File-Adaptive-Clustering".

There are two limitations:
1. Files over 20 MB won't be defragged (puts too much of a hit on performance to move such a big file during file access?). To defragment these, you'll likely have to run a third party disk utility.

2. Files that are not being accessed are not defragmented.

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