There are two automatic file optimization routines that take place in Panther. Both take place only on the boot volume and require the use of HFS+ (Mac OS Extended) disk format and that journaling be turned on.
Automatic File Defragmentation.
When a file is opened, if it is highly fragmented (ie. 8+ fragments) and the file is under 20MB in size, it will be automatically defragmented. This is accomplished by the file system just moving the file to a new location. While this does defragment relatively small and badly fragmented files, it tends to contribute to more disk/volume fragmentation, a performance tradeoff.
Adaptive Hot File Clustering
On drives over 10 GB in size for a period of 60 hours, the file system keeps track of files that are read frequently (for a file to be considered as a hot-file, it must be less than 10MB and never written to - in other words system and application executable files). At the end of this period, the the files that have been read the most times are moved to the "hotband" of the disk. The "hotband" is that part of the disk which is particularly fast given the physical characteristics of the disk and will vary from brand to brand and model to model.
The size of the "hotband" depends on the size of the disk and is allocated at the rate of 5MB of hotband space for each GB of disk. So a 120 GB Hard Disk would have a "hotband" of 600 MB. "Cold" files that were in the hotband will be moved out of the hotband to make room for the hot files. As a side effect of being moved into the hotband, the hot files are defragmented.
Question
If Panther optimization is as good as the slashdot article would indicate, why is it that my boot drives tend to become extremely fragmented over time? I will also admit that as long as there is plenty of free space on a drive, I have not observed any noticeable performance benefit from defragmentation. On a crowded drive with minimal free space, that is another story altogether.
