Should APFS Be Used on All Disk Types?

Is your disk a good candidate for APFS?

Disk Utility and APFS file system

Coyote Moon, Inc.

APFS (Apple File System) is a new file system optimized for SSDs (Solid-State Drives) and Flash devices such as USB thumb drives. And although it is geared towards the physical characteristics unique to flash-based storage, it is also being targeted as a universal file system replacement for any storage device.

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 being used with just about any storage system, including optical discs, USB thumb drives, solid state drives, and platter-based hard drives.

It's the versatility of the macOS and all of the storage system options available to it that has us asking this question: Should APFS be used on all of the disk types supported by the macOS?

Which Types of Disks Are Best Suited for Use With 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 as the file system to use.

Let's take a look 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 efficiencies of storage leading to more free space being available. The storage space gains come from features built-in to APFS including:

  • Clones that are created almost instantaneously 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 that 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 unrealistically 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 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 and when the operating system was publicly released support for APFS on Fusion drives was pulled, and the operating systems disk utility was modified to prevent Fusion drives from being converted to APFS format.

Speculation initially 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 write process, it can also lead to a great deal of file segmentation, scatter 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 be happening 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 of course moving less often used files from the SSD to the hard drive. All this copying could have cause fragmentation issues on the hard drive when APFS and Copy-on-Write were being used.

Apple has promised that APFS will at some future release be ready for use with Fusion and tiered storage systems, which leaves us with 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-in to the APFS system.

Apple's goal for APFS on a hard drive was to be neutral, that is the user should not see much in the way of overall performance improvements, but certainly not see any obvious degradation of performance. 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.

Where an issue may pop up is when extensive edits are performed routinely, such as those editing images and videos on a regular basis, or someone working with audio, creating podcasts, or editing music. 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 could occur when APFS is used on hard drives used in an extensive media editing environment.

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 a performance degradation over time as the drives become fragmented.

APFS on Externals

  • Maybe not a good idea – If your external drives (including USB thumb drive) 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 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.