Even by the standards of recent macOS releases, this year’s High Sierra is shaping up to be a low-key release with few high-profile user-visible improvements. Apple’s highlight page covers quite a few things, but in most cases they’re iterative tweaks that would mostly belong in the “grab bag” section of an overview of, say, Leopard or even Yosemite. Don’t get me wrong, I’m looking forward to iCloud-backed iMessages and family iCloud storage plans, but support for tables in Notes and flight status updates in Spotlight aren’t exactly life changing (not unless your life is continuously interrupted by extremely small and specific problems).

But to call High Sierra a minor release is to ignore the big under-the-covers changes it brings to the Mac, some of which have been in the works for years now. New filesystems and graphics APIs may be hard to demo to more casual users, but there’s plenty in this release that lays the foundation for more visible changes somewhere down the line.

In lieu of a traditional preview of High Sierra, we’ve browsed the dev docs and talked with Apple to get some more details of the update’s foundational changes.

APFS

Longtime followers of Ars Technica’s macOS reviews will know that a new filesystem has been a long time coming. The current HFS+ is more than three decades old, and it’s showing its age despite all the features Apple has bolted onto it over the years.

Like iOS 10.3, High Sierra will convert your boot drive to APFS when you first install it—this will be true for all Macs that run High Sierra, regardless of whether they’re equipped with an SSD, a spinning HDD, or a Fusion Drive setup. In the current beta installer, you’re given an option to uncheck the APFS box (checked by default) before you start the install process, though that doesn’t necessarily guarantee that it will survive in the final version. It’s also not clear at this point if there are edge cases—third-party SSDs, for instance—that won’t automatically be converted. But assuming that most people stick with the defaults and that most people don’t crack their Macs open, most Mac users who do the upgrade are going to get the new filesystem.

If you prefer to be more cautious about things, Apple tells us that HFS+ isn’t going anywhere anytime soon. High Sierra can continue to boot from HFS+ partitions, and Disk Utility will retain the ability to format and work with HFS+ drives. It’s not clear when that will change—Apple may begin adding features that work best with (or work specifically because of) APFS, as it already appears to be doing in iOS 11—but it won’t be with this release.

When you installed the iOS 10.3 update, you may have noticed that the filesystem conversion meant that it took quite a bit longer to complete than a regular iOS update. That’s going to be true of the macOS conversion, too. Apple didn’t have specific time estimates to give me, but in the end it’s going to vary widely based on the size, type, and speed of your disk, the speed of your Mac, and how much data you have stored on it.

We've drilled down into some of the more esoteric technical aspects of APFS in other pieces (at least, APFS in its current Sierra beta form), so what I want to focus on here is what APFS is actually letting Apple do in macOS. One of the more obvious benefits, and the one Apple showed off on stage during its keynote, is the ability to copy files on the same disk without actually physically storing two different copies on the disk. Native support for solid-state drives and encryption is harder to quantify, but is nevertheless important given the increasing prevalence of both. But what Apple stressed to me in particular is how much more flexible APFS can be, both with current technologies and when it comes time to implement future tech.

Let’s take Fusion Drives as a good example, a feature announced in 2012 to solve what was (then) a new problem: SSDs were fast but small and expensive, while HDDs were large and cheap but really slow. Combining the two to get the best of both worlds totally made sense, and still does in desktops despite the increased capacity and lower cost of SSDs in 2017.

HFS+ has no idea what a Fusion Drive is. It can’t distinguish an SSD from a spinning HDD, nor is it even aware that Fusion Drives are actually two disks presented to the operating system as a single volume. Apple relies on an underlying technology called Core Storage to do the heavy lifting here—it’s Core Storage that combines the drives and presents them to the OS and the filesystem as a single disk, and it’s Core Storage that’s responsible for shunting data to and from your SSD depending on what apps and files you’re accessing.

But Core Storage has its own limitations. It’s not a filesystem, and it’s not actually aware of files—all Core Storage sees are blocks, and a block that contains essential system files looks the same as a block that contains, say, an application, or a spreadsheet. Fusion Drive keeps track of how often blocks are accessed, and it moves frequently accessed blocks to the SSD while keeping infrequently used blocks on the spinning disk.

APFS can avoid all of this. Like HFS+, it’s aware of files and capable of telling the difference between them, but like Core Storage it can also handle combining drives and moving data around in the background. That’s going to make Fusion Drives a lot smarter in High Sierra than they currently are.

For instance, APFS can tell the difference between different types of files. If you’re frequently accessing something that takes up a lot of space but doesn’t really benefit from extra storage speed—a video file, for instance—APFS can decide not to move that file to your SSD, freeing up more space for other things and reducing the amount of wear on the drive. APFS can also keep file metadata on the SSD while keeping files themselves on the spinning disk, speeding up Spotlight searches and quick trips to “Get info” windows.

APFS also has benefits for local Time Machine snapshots, something that can normally take up quite a bit of space. Because of APFS’ support for snapshots, Time Machine no longer has to save multiple full copies of a file to your disk—it can just keep track of the specific changes. If you’re editing a PowerPoint presentation, for instance, changing an individual slide with HFS+ means saving two copies of the file, one that records your new changes and one just in case you want to revert. Now, it can just save the original file plus a record of the differences between the original file and any updated versions, accomplishing the same task in substantially less space. And as with the Fusion Drive improvements, taking up less space on SSDs in particular means writing less data to the drive, which ultimately means a drive that’s going to last longer.

Apple says these changes won’t be extended to normal Time Machine backups, mostly because of the number of variables at play—the filesystem of the target drive, the file sharing protocol being used for backups to networked storage, and so on—but APFS at least opens the door to more efficient, more granular Time Machine backups at some point in the future.

That’s not everything that APFS does, but it’s a pretty good illustration of how Apple is planning to use it to modernize core Mac technologies behind the scenes. Most people couldn’t tell you what filesystem their computer was using if they were even aware of what filesystems were in the first place, but that doesn’t mean they won’t see the benefits from changes like these.