APFS Explained: What You Need to Know About Apple’s New File System

Apple’s macOS 10.13 High Sierra brings a new file system named “Apple File System”, which largely replaces the older HFS+ file system. Apple File System, also known as APFS, has been used by default on iPhones and iPads since iOS 10.3, and is also used on the Apple Watch and Apple TV—but now it’s finally on the Mac, too.

How to Get the Apple File System

You don’t have to do anything special to switch your Mac to the new APFS file system. Just upgrade to macOS 10.13 High Sierra. The upgrade process will automatically migrate your Mac’s internal drive from HFS+ to APFS, assuming your Mac’s internal drive is an SSD or another all-flash storage device.

This process is automatic. On a Mac with all flash storage, the internal partitions will be migrated from HFS+ (also known as “Mac OS Extended”) to APFS. There’s no way to opt out of this conversion.

Fusion Drives (which incorporate both flash and traditional magnetic storage), traditional hard disk drives, and non-Mac volumes (like Windows Boot Camp volumes) won’t be migrated. While APFS doesn’t work on Fusion Drives at the moment, Apple plans to enable APFS on Fusion Drives in the future.

External drives, including USB drives and SD cards, will also not be migrated to APFS. You can choose to format an external drive as APFS with Disk Utility. However, other file systems are recommended. For example, exFAT offers greater compatibility with Windows and other devices. Mac OS X Extended offers compatibility with High Sierra as well as Macs running older versions of the macOS operating system.

Your Time Machine drive can’t be formatted APFS yet, either. Time Machine can back up from an APFS drive, but the Time Machine destination drive must be formatted as HFS+. The operating system will handle this all automatically—just don’t try to convert your Time Machine disk manually and you’ll be fine.

The Benefits of APFS

So why care about APFS at all? It comes with a number of advantages over HFS+, most notably performance and reliability, with a few improvements to encryption and partitions as well.

Performance Increases

You won’t see any new whiz-bang features with a new file system, but you will see a variety of under-the-hood improvements. For example, you’ll see improved performance in some file operations.

Faster directory sizing may actually be noticeable. When you click the “Get Info” button for a large folder, you’ll see the folder’s total file size noticeably sooner. That’s because APFS stores metadata about the sizes of files in a place where it can be accessed more quickly, whereas HFS+ made the operating system examine the metadata of each individual file one by one.

Copying files will also be faster. Let’s say you copy a file from one folder to another. Rather than simply creating a second copy of that file’s data on the disk, APFS creates a marker that says there are two files on the disk that point to the same data. This means that the copy operation should happen immediately. If you modify one of the two files, APFS will store both the original and changed file, and everything will work like you expect it to. It’s just faster and more efficient under the hood.

Performance when creating “sparse files” is also improved. In other words, if an application creates a large file that’s empty, this is now much faster. With HFS+, an application creating a 5 GB file would have to wait while the operating system wrote 5 GB of zeroes of the disk. With APFS, the file system marks the space as allocated but doesn’t write to it immediately, so this should now be nearly instantaneous.

Reliability and Data Integrity Improvements

Apple’s new file system is more resistant to data corruption due to bugs and power failures, too.

APFS uses “copy-on-write”. For example, when you update a file’s metadata—like its file name, for example—the HFS+ file system will directly modify that metadata. If your Mac crashes of the power goes out before the operation is finished, data corruption may occur. With APFS, when you modify a file’s metadata, APFS will create a new copy of the metadata. APFS points the original file at that metadata only after the new metadata is written, so there’s no risk of the metadata being corrupted. This feature is also found in other modern file systems, like ZFS and BtrFS on Linux and ReFS on Windows.

Apple File System also uses something called “Atomic Safe-Save”, which is like copy-on-write but applies to other file operations, including renaming a file or moving it.

Reliability is also improved thanks to APFS creating and storing checksums associated with data on the disk. When APFS writes a file to disk, it examines the file, runs it through a mathematical formula that generates a shorter string that matches the file, and writes that to disk as well. When APFS reads data, it compares the data to the checksum on disk and verifies it matches. If the data doesn’t match the checksum on disk, this indicates data corruption. It could be due to a bug, hardware failure, or something else—but the operating system can recognize it immediately.

Other New Features

This file system also lays the groundwork for new features and other improvements, which can build on what APFS offers.

For example, APFS incorporates snapshots at the file system level. The first snapshot contains a complete picture of the entire drive, while future snapshots only contain the changes made since the previous snapshot. Only new data you’ve added takes up space. Time Machine works similarly, but APFS’s snapshots are even more efficient. Time Machine doesn’t yet use APFS, but Apple could move Time Machine to APFS in a future release of macOS.

APFS also supports multi-key encryption, which allows for different keys used to encrypt different data on the hard drive. The way FileVault encryption works hasn’t changed yet, but one day macOS could use different encryption passphrases for each user’s data and the system data.

“Space sharing” is one new feature some people will benefit from today. Traditionally, if you created multiple volumes (partitions) on one physical disk, you had to decide up front how much space each volume would get. So, you might create five different 100 GB volumes on a 500 GB drive. If any of those volumes needed more than 100 GB of space, you’d have to manually resize the volumes. However, if one volume just needed 20 GB of space, you’d have 80 GB of space wasted—unless you resized the volume and then allocated that space to another volume. With APFS, you could create five volumes on a 500 GB drive and not worry about how much each one needs. The volumes will share space. As long as the total space used by those five volumes is less than the 500 GB of total available space, things will just work.

More technical information about APFS is available on Apple’s developer website.

What Do I Need to Know About Using APFS?

The switch to APFS should be largely transparent. Your drive will be migrated automatically if APFS supports it. Time Machine and File Vault still work normally.

Some problems do exist with Boot Camp, however. A Windows system installed alongside macOS can’t yet read APFS, even with Apple’s Boot Camp software installed. This means you can’t use the Boot Camp Control Panel to change your startup disk from within Windows at the moment. To reboot back into macOS, hold down the Option key while booting your PC and choose macOS. You can still control your startup disk from System Preferences > Startup Disk in macOS. Apple will hopefully fix this at some point soon.

When using the Disk Utility application (available at Finder > Applications > Utilities > Disk Utility), you’ll likely see that your Mac’s drive is APFS (unless it’s a Fusion Drive or mechanical hard drive which wasn’t migrated).

Thanks to space sharing, even if you have a single volume (partition) on your drive like most people, your drive is formatted with an APFS container that can hold multiple volumes. That’s why you’ll see that it’s shared by multiple volumes here.

To add a new volume, click the “New Volume” button. This will add new volumes to the larger APFS container. They’ll appear just like normal volumes or partitions in Finder and elsewhere on the system, but they’ll share space with all the other volumes in the APFS container.

Don’t use the “Partition” button to add a new partition unless you want to add a new, non-APFS volume to your system. Adding a new partition will take space away from the APFS container. However, it is mandatory when adding a Windows volume for Boot Camp, for example.

You still have some control over the size of your APFS volumes. When creating a new APFS volume, you can click the “Size Options” button and specify a reserve size (minimum size) and quota size (maximum size) for the volume, ensuring it won’t become too small or too large. This isn’t necessary, of course—APFS works automatically even if you don’t specify these options. They just exist if you want that extra control.

The switch to APFS won’t be noticed by most Mac users, but it lays the groundwork for future improvements, boosts performance in some situations, and helps protect against data corruption. It also moves macOS to the same file system already in use by Apple’s other operating system, iOS.

Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.