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

Aside From a Few Specific Uses, Defragmenting May Not Be Necessary

Drive Genius 4 Defragment feature
Fragmentation levels displayed using Drive Genius 4. 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 any of the drives connected to your Mac. 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 as well as macOS have their own built-in safeguards that prevent files from becoming fragmented in the first place.

  • The Mac'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.
  • The Mac OS 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 and MacOS implement 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 startup drive. In the process of moving these files, OS X defragments them, and then stores them in the area of the drive that has the fastest access.
  • When you open a file, the Mac checks to see if it is highly fragmented (more than 8 fragments). If it is, the operating system will automatically defragment the file.

The result of all these safeguards is that the Mac 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, the Mac operating system 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 your Mac 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 recording and editing, complex scientific data acquisition, or working with time-sensitive data.

This only applies to standard hard drives. If you’re using an SSD, or a Fusion drive, its data should never be defragmented, as doing so can lead to write amplification, a common cause of premature failure of the SSD. SSDs have a finite number of writes that can be performed. You can think of it as the memory location within the SSD becoming brittle with age. Each write to a memory location increases the age of the cell.

Because flash-based storage requires the memory locations to be erased before new data can be written to them, the process of defragging an SSD can lead to multiple write cycles, causing excessive wear on the SSD.

Question: Will Defragmenting Harm My Drive?

As we mentioned, defragmenting an SSD or any flash-based storage device (this includes Fusion-based drives that use a small SSD/flash device along with a standard hard drive) can lead to premature failure by increasing the amount of wear (writing and reading of storage cells). In the case of a hard drive, one that uses a mechanical rotating platter, there's no significant chance of damage to the hard drive, or to your Mac, simply by performing a defrag. The only negative comes in the time it takes to perform the defragmentation.

What If I Decide That I Do Indeed Need to Defragment?

There are third-party utilities available that can defragment your Mac's drives. One of my favorites for this task is:

Drive Genius 4: Tom's Mac Software Pick

Drive Genius 4 does a lot more than offer the ability to defragment a Mac's drive; it includes the ability to monitor drive health as well as repair most drive problems.

Reference

Hot File Adaptive Clustering

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