As a Certified Apple Computer technician I often get ask questions about defraging and whether its necessary on a Mac. The question isn’t as silly as it first might sound. The simple answer is yes.
With Mac OS X and its file system, there is a small amount of automatic defragmentation that happens, however, this only happens in files over a specific size and the type of defragmentation is not the same as what you would get with a specialised program. More about this later.
So what does the built in defragmentation safeguards actually do?
The file system will not use recently freed up space to write new files, it will instead go to the end of the disk where it finds a large enough space for the entire file it is seeking to write.
It also has the ability to group together large amounts of small files into one spot so that it leaves larger chunks of free space on your drive which it might need for larger files in the future, hence reducing the need for defragmentation of files.
When you open a file, Mac OS X will check if its fragmented, if it has more than 8 fragments, it will defrag the file. The OS will also do a small amount of monitoring of files that do not get changed (read only), but are often used, to spots on the Hard Disk that are quickest to read off.
So why then do I need to defragment my drive is the OS does it automatically?
While the OS have some great built in safeguards to keep your drive as not fragmented as possible, it doesn’t go as far as a specialised program would. If Apple implemented all the features and techniques that these programs use, it would greatly reduce write speeds on the machine and increase the amount of reading and writing of files causing a slow down in operation and an increased load on the drive.
Specialised tools will do better job of defragmenting your drive by grouping your most frequently used files on the fastest part of the drive, defragmenting all files regardless of size, all resulting in an highly optimized drive. Repairs can also be made on some files, like cross-linked files during this optimization process, which the automatic features don’t address.
Other features a specialized tool can offer over the built in features include. Thermal monitoring of the Hard Disk, multiple defrag algorithms for better optimization depending on the type of fragmentation that’s being caused as well as offering support for automate such optimization on a scheduled basis to keep mission critical drives operating and their peak.
It is therefore still a required procedure, although you can get away with it for longer before noticing a slow down in performance. It is also important to note that all the these automatic features of the OS and file system do no work when you have less than 10% free space.