Apple File System for macOS optimizes access to 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 Apple operating systems use only solid-state storage systems, macOS is capable of using any storage system, including optical discs, USB thumb drives, solid-state drives, and platter-based hard drives.

Which Types of Disks Are Best Suited for APFS?

Because APFS was originally designed for use with SSDs and flash-based storage, it would seem that the new file system would be at home on these newest and fastest storage systems. APFS works with most storage systems, 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

Starting with macOS High Sierra, SSDs used as startup drives are converted automatically to APFS when the OS upgrades—including internal SSDs and external SSDs connected using Thunderbolt. USB-based external SSDs are not automatically converted, though you can manually convert these to APFS.

In testing, APFS showed improved performance and gains in storage efficiency, leading to more available free space. The storage-space gains come from features built into APFS, including:

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

  • Clones: Clones generate almost instantly without taking up any significant additional storage space.Space Sharing: Several volumes share the free space within an APFS container.Copy-on-Write: Data structures share when no changes are present.Sparse Files: These files offer a more efficient way to manage free space.

APFS speed gains with solid-state drives are seen in boot time, which has shown dramatic improvement, and with file copying, which due to cloning, can be fast.

APFS on Fusion Drives: No

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 the Apple tiered storage solution, the Fusion Drive. Fusion Drive is a combination of a small but 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.

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

Speculation pointed to a reliability issue with converting existing Fusion Drives to the APFS format. But the real challenge 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 approach protects data during the writing process, it can also lead to 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 often happen because 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 glitches on the hard drive when APFS and Copy-on-Write are in use.

APFS on Hard Drives: Maybe

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

Apple’s goal for APFS on a hard drive was to be neutral. There’s not 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 performance problems.

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

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, and working with images, these use cases work fine on an APFS-formatted hard drive.

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

The Fusion Drive and Copy-on-Write challenge could lead to disk fragmentation. The same situation occurs when APFS is used on hard drives that are used for media production.

Most people performing this type of resource-intensive work have likely moved their Mac to an SSD-based storage system. However, there are some 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

APFS-formatted drives currently can only be accessed by Macs with the Sierra operating system or newer. If you intend to share data on an external drive with several computers—especially Windows and Linux machines—leave the drives formatted in a common file system, such as HFS+, FAT32, or ExFAT.

Time Machine Drives: No

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

If your external drives (including USB thumb drives) are shared among several Macs or computing platforms, don’t convert those drives to APFS.

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

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