Should APFS Be Used on All Disk Types?

Is your disk a good candidate for APFS?

APFS (Apple File System) is a file system for macOS optimized for SSDs (Solid-State Drives) and Flash devices such as USB thumb drives.

APFS is used on all Apple operating systems including watchOS, tvOS, iOS, and macOS. While most of the Apple operating systems use only solid-state storage systems, the macOS is capable of using about any storage system, including optical discs, USB thumb drives, solid-state drives, and platter-based hard drives.

The versatility of macOS and other storage systems begs the question: Should APFS be used on all of the disk types supported by macOS?

Disk Utility and APFS file system.

Which Types of Disks Are Best Suited for APFS?

Since APFS was originally designed for use with SSDs and flash-based storage, it would seem obvious that the new file system would be right at home on these newest and fastest storage systems. For the most part, you would be correct, but there are specific usages that can make APFS a poor choice, or at least a less than optimal choice.

Let's take a look at how suitable APFS is for common disk types and usage.

APFS on Solid State Drives: Yes

  • APFS on solid-state drives brings out the best features of the new file system.

Starting with macOS High Sierra, SSDs used as startup drives are converted automatically to APFS when the OS is upgraded. This is true of internal SSDs and external SSDs connected via Thunderbolt. USB based external SSDs are not automatically converted, though you can manually convert them to APFS if you wish.

APFS is optimized for solid-state drives and flash-based storage systems such as USB thumb drives. In testing, APFS showed improved performance as well as gains in storage efficiency, leading to more free space being available. The storage space gains come from features built into APFS, including:

  • Clones: These are created almost instantly without taking up any significant additional storage space.
  • Space Sharing: Allows multiple volumes to share the free space within an APFS container.
  • Copy-on-Write: Allows data structures to be shared when no changes are present.
  • Sparse Files: A more efficient way to manage free space.

APFS speed gains with solid-state drives are seen not only in boot time, which has shown dramatic improvement but also with file copying, which thanks to cloning can be quite fast.

APFS on Fusion Drives: No

  • Fusion Drives are not a candidate for upgrading to the APFS system.

It seemed the original intent of APFS was to work seamlessly with both hard drives and SSDs. During the initial beta versions of macOS High Sierra, APFS was available to install on SSDs, hard drives, and on Apple's tiered storage solution, the Fusion Drive. Fusion Drive is a combination of a small but very fast SSD along with a large but slow hard drive.

Fusion Drive performance and reliability with APFS seemed to come into question during the betas of macOS High Sierra. When the operating system was publicly released, support for APFS on Fusion drives was pulled, and the operating system's disk utility was modified to prevent Fusion Drives from being converted to APFS format.

Speculation pointed to a reliability issue with converting existing Fusion Drives to the APFS format. But the real issue may be a performance hit taken by the hard drive component of the Fusion pair. One of the features of APFS is a new technique to ensure data protection called Copy-on-Write. Copy-on-Write keeps data loss to a minimum by creating a new copy of any file segment that is being modified (write). It then updates the file pointers to the new copies after the write is successfully completed. While this ensures data is protected during the writing process, it can also lead to a great deal of file segmentation, scattering parts of a file around a disk.

On a solid-state drive, this is not much of a concern. On a hard drive, it can lead to disk fragmentation and reduced performance. On a Fusion drive, file copying can happen often since one of the functions of tiered storage is to move frequently used files from the slower hard drive to the faster SSD, and rarer files from the SSD to the hard drive. All this copying could cause fragmentation issues on the hard drive when APFS and Copy-on-Write are in use.

Apple has promised that APFS will at some point be ready for use with Fusion and tiered storage systems. This leads us to the question: How well does APFS work with a standard hard drive?

APFS on Hard Drives: Maybe

  • APFS can be used on hard drives, though the benefit is small.

You may want to use APFS on your hard drives if you are using File Vault to encrypt your drive. Converting to APFS will also replace the File Vault encryption with the far more robust encryption system that is built into the APFS system.

Apple's goal for APFS on a hard drive was to be neutral. The user should not see much in the way of performance improvement, but also not much degradation either. In essence, APFS on a hard drive should provide for a general improvement in data safety and security without imposing any obvious performance issues.

It appears, for the most part, APFS has met this neutral performance goal for hard drives, though there are some areas of concern. For general computing usage such as working with emails, writing office documents, browsing the web, performing basic research, playing a few games, listening to music, watching videos, working with images, and videos should all work fine on an APFS-formatted hard drive.

Issues may occur when performing extensive edits, such as in photo, video, or audio software — any activity where large-scale file editing is being performed.

Remember the Fusion Drive and Copy-on-Write issue that could lead to disk fragmentation? The same issue can occur when APFS is used on hard drives that are used for media production.

Ideally, anyone performing this type of work has likely already moved their Mac to an SSD-based storage system. But there are still quite a few who may be using hard-drive-based RAID storage systems to meet their editing needs. In that case, APFS and Copy-on-Write may cause performance degradation over time as the drives become fragmented.

APFS on Externals: Probably Not

  • If your external drives (including USB thumb drives) are shared among multiple Macs or computing platforms you will want to avoid converting them to APFS.

APFS formatted drives currently can only be accessed by Macs running the Sierra or High Sierra operating system. If your intent is to share data on an external drive with multiple systems, it is best to leave the drives formatted in a more common file system, such as HFS+, FAT32 or ExFAT.

Time Machine Drives: No

  • Time Machine requires drives formatted with the HFS+ file system.

If you were to convert a Time Machine drive to APFS the Time Machine app would fail on the next backup. In addition, the data on the Time Machine drive would have to be erased to format the drive back to HFS+ for use with Time Machine.