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What Apple’s Forthcoming APFS File System Means to You

Among the tidbits Apple revealed to its developer audience at the recently completed Worldwide Developers Conference was a new file system for the whole range of its products (see “macOS 10.12 Sierra to Succeed OS X 10.11 El Capitan,” 13 June 2016). Dubbed “APFS” (an acronym that Apple doesn’t completely spell out even in its developer documentation), the file system is meant to replace HFS+, the file system that in turn replaced 1985’s HFS (Hierarchical File System) in 1998. (HFS+ has received numerous updates since 1998, so don’t get the impression that it’s completely obsolete.) Apple released a developer preview of APFS with macOS 10.12 Sierra, and the company says APFS will become the default file system in all of its operating systems — macOS, iOS, watchOS, and tvOS — by late 2017.

Changing the default file system for an operating system is a big deal, since the file system is responsible for keeping track of all of the data on the device. But what does such a change mean for users?

The Finder Is Not the File System — Unless you’re one of those rare individuals who lives on the Terminal command line and who can type ls -la faster than you can double-click a folder icon, the Finder, along with the Mac’s Open and Save File dialogs, is normally how you see what’s on your Mac and how you navigate among your files and folders. This won’t change when APFS takes over from HFS+.

That’s because the Finder is a client of the file system. The Finder shows you a view of the items you have stored on your Mac and lets you arrange files and folders in a manner that works for you. The Finder, along with its folder and file icons, has been on the Mac since before even HFS came around, and has worked more or less the same from the user’s point of view since the first Mac came out of a bag and said “Hello” back in 1984.

The file system works behind the scenes, providing information to the Finder and to applications about the files and directories that are stored in some fashion on a device connected to your Mac — whether in magnetic fields recorded on a spinning platter or charged cells in a solid-state device. The file system keeps track of how much storage capacity files take up on the device, where on the device the data that make up the files are stored, and all sorts of metadata about those files, such as their names, when they were created and last changed, which users are allowed to open them, and a great deal of other stuff.

The Finder is thus an intermediary. Its job is to present us ordinary mortals with an easily understandable view of the data that the file system actually manages, and to instruct the file system about what you want to do with that data. As long as the Finder can communicate adequately with a file system to enable that view and pass along your instructions, the actual file system being used on a storage device doesn’t matter much.

In fact, if you have ever used a thumb drive or an SD card with your Mac, you may have already seen the Finder working with a file system other than HFS+. Most thumb drives and SD cards come formatted for Windows computers and use the FAT (File Allocation Table) file system, and yet you can still see and manipulate the files and folders on them with the Finder.

APFS is designed to understand nearly all of the same instructions and information requests that the Finder, or any other Mac application, issues regarding storage devices managed by HFS+. You’ll still be able to move files, rename files, copy files, open files, delete files, tag files, and so on just as you always did.

But You Will Notice Some Differences — However, APFS does provide some benefits over HFS+, and they’re significant enough that you will likely notice them in action.

HFS+ came along well before large storage devices containing gigabytes, let alone terabytes, were common, well before flash memory was commonly used for file storage, well before file encryption was something that ordinary users cared about, and well before Mac OS was replaced with OS X and its virtual-memory-enabled multi-tasking capabilities.

Support for large data volumes, encryption, flash drives, and virtual memory was more or less bolted onto HFS+ instead of being integrated into it. APFS has that support, and more, built in and will thus have an impact on your experience as a user.

  • Faster file copying and saving: Saving files and duplicating them on the same device will happen much more quickly and without using as much space. That’s because APFS has been designed to reduce data duplication by sharing data between files as necessary. When you copy a file to another folder in APFS, no data is moved. Instead the copy points to the original file’s data on the device. If one of those copies is later changed, only the changes are stored: APFS handles which bits belong to which files. APFS’s capability to change only parts of files also speeds up file saving, and, especially, automatic saving using versions. Viewing and restoring from a previous version in an app should be faster too.

    Apple’s goal of optimizing APFS for flash storage and solid-state storage devices (SSD for short) is behind this shared data approach. When you save a file repeatedly over time, the actual data making up the file can’t be stored in one contiguous chunk on the drive, and thus it becomes “fragmented.” That’s normal, but on a spinning disk, file fragmentation reduces performance by forcing the read/write heads to seek more in order to access the widely separate parts of the file. On a spinning disk, the shared data approach will mean more disk fragmentation.

    But fragmentation on an SSD does not affect speed nearly as much. On an SSD, rewriting memory cells is much more costly, both in terms of speed and in terms of the cells’ usable life: rewriting SSD cells gradually wears them out. Writing just the changed file contents, and doing so in scattered physical locations on an SSD, is an efficient way to increase wear-leveling as well as to reduce the number of write operations. That’s a win-win now that most Apple devices, including Macs, rely on flash storage.

    However, APFS is designed to work with spinning disk drives as well and should be smart enough to take disk location and rotational latency into account when choosing locations to store file changes on such devices.

  • Snappier backups and restores: APFS can create read-only snapshots of a volume’s file contents. Once again, this capability comes courtesy of the shared data approach. Since file changes are scattered in different locations on a device, APFS can keep track of the data locations that comprise a file at any given time and make sure they are preserved. New versions of Time Machine, as well as other backup utilities, should be able to take advantage of APFS snapshots, making incremental backups that much more snappy.

  • Flexible partitions: APFS creates physical containers on a device, which can then be subdivided, or partitioned, into individual storage volumes. Unlike the partitions managed by HFS+, APFS partitions share their space with each other within the same container. This means that if you have a 500 GB APFS container, you can create multiple volumes within it that each will show up in the Finder as having 500 GB capacities. As a consequence, the amount of free space shown for each volume reflects the free space available in the container, not the individual volumes.

    Although this approach could lead to user confusion, it might be mitigated by one other APFS feature: fast directory sizing. Under APFS you will no longer have to wait for seconds or even minutes while the Finder shows you the exact amount of storage used by a folder in a Get Info window. Fast directory sizing makes that information much more quickly available — useful if you want to know if your photo library will fit on the portable drive you’re bringing on your vacation.

  • More encryption options: macOS Sierra, like versions of OS X since 10.7 Lion, offers full disk encryption via FileVault 2. Similarly, although this isn’t obvious to the user, iOS has offered individual file encryption since iOS 4 and has encrypted all user files created by third-party apps since iOS 7. APFS provides both full-volume and individual file encryption: you can use full volume encryption on one volume in an APFS container and use individual file encryption on another volume in the same container.

    In addition, APFS provides not only single-key file encryption but multi-key encryption, allowing a file’s data and its metadata to be encrypted separately. For example, you could have a volume that allowed a file cataloging utility to decrypt metadata about a file, such as its name and creation date, but not to decrypt the actual file contents.

    Regardless, although it’s difficult to know how Apple and other developers will take advantage of these capabilities, it’s safe to say that you’ll have more encryption options available to you when APFS is in charge of your device storage. Plus, since Apple built encryption into APFS, encryption should be snappy.

Transition to APFS — Apple intends the transition to be as painless as possible. When you buy a new Mac once APFS is the default file system, you should have no transition hassle at all: Setup Assistant should move your data from your old Mac to the new one pretty much as it has all along. And, since macOS will continue to support HFS+ as well as APFS (just as it supports other file systems), you’ll be able to mount and use older HFS+-formatted external drives with no problems.

Apple also plans to provide an in-place APFS migration utility for users who upgrade older Macs to a macOS version that uses APFS as its default. Most likely, the migration process will take some time (minutes or hours) to convert an existing HFS+ device to APFS, but Apple’s goal is to make the process as simple and as safe as possible.

Let’s all hope Apple meets that goal: APFS is a big deal, offering notable performance, space-saving, and security benefits to users while promising to work its magic unobtrusively behind the scenes — as any good file system should.

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