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Do You Need to Defragment a Mac’s Hard Drive?

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Defrag a Mac's drive

Fragmentation levels on your Mac's startup drive usually remain low.

Screen shot courtesy of Coyote Moon, Inc.

Question: Do You Need to Defragment a Mac’s Hard Drive?

I’m a Windows user who recently made the change to Apple and the Macintosh. I’m used to routinely defragmenting my hard drive in order to ensure top performance by my computer. I don’t see any way to defragment my Mac’s hard drive. Do I need to be concerned about this?

Answer:

Apple supplies a handy application for working with hard drives called Disk Utility. If you open up Disk Utility, you’ll notice that it doesn’t include a tool for defragmenting your hard drive. The reason for this perceived oversight is that a Mac running any version of OS X later than 10.2 does not need to be defragmented. OS X has its own built-in safeguards that prevent files from becoming fragmented in the first place.

  • OS X’s HFS+ file system tries not to use recently freed file space on a disk. Instead, it looks for larger free areas already present on the drive, thereby avoiding fragmenting files just to fit them into available space.

  • OS X dynamically gathers groups of small files and combines them into larger areas on your disk automatically. The process of writing the files to a new larger location defragments all of the files in the group.

  • OS X implements Hot File Adaptive Clustering, which monitors frequently-accessed files that do not get changed (read only), and then moves these often-accessed files to a special hot zone on the hard drive. In the process of moving these files, OS X defragments them, and then stores them in the area of the hard drive that has the fastest access.

  • When you open a file, OS X checks to see if it is highly fragmented (more than 8 fragments). If it is, OS X will automatically defragment the file.

The result of all these safeguards is that OSX rarely, if ever, needs to have its disk space defragmented. The only real exception to this is when your hard drive has less than 10 percent free space. At that point, OS X is unable to perform its automatic defragmentation routines, and you should consider either removing files or expanding your disk storage size.

Question: Is There Any Reason Not to Defragment My Mac's Drive?

As we mentioned above, you probably don’t need to defragment your drives, because OS X takes care of that for you. However, there are some types of tasks that can benefit from defragmented drives; specifically, when working with real-time or near real-time data acquisition or manipulation. Think video or audio editing, complex scientific data acquisition, or working with time-sensitive data.

This only applies to standard hard drives. If you’re using an SSD, its data should never be defragmented, as doing so can lead to premature failure of the SSD.

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