A disk image is a file that contains an exact, byte-for-byte replica of an entire storage medium—whether that’s a CD, DVD, USB drive, or full hard disk.
This includes not just files and folders, but also the system's file structure, boot sectors, and even empty space. Essentially, it's like a digital clone, preserving everything from system data to personal files in a portable, restorable format. Think of it as bottling the essence of a disk into one neat package.
Operating systems use disk images for several strategic reasons.
First and foremost, installation and deployment—when users install Windows or Linux, they're usually booting from an image file, such as an ISO, that contains all the files needed to set up the new operating system. Disk images also make system recovery and backup fast and efficient. By capturing an entire operating system state in an image, users can restore it quickly in the event of corruption, malware, or hardware failure.
They're also widely used in virtualization, where operating systems run inside disk images managed by hypervisors like VirtualBox, VMware, or QEMU. This makes testing, sandboxing, and multi-environment setups easier and cleaner.
And here, Apple has introduced a new format into the mix it calls the ASIF (Apple Sparse Image Format).
Disk images have long been a staple of macOS life.
From encrypted volumes to managing virtual machines, formats like UDSP (sparse disk images) have offered flexibility—but at the cost of speed. Encrypted sparse images, in particular, have suffered from sluggish performance, often barely crossing 100 MB/s, even on Apple's blistering-fast SSDs.
That changes.
Through the announcement of macOS 26 "Tahoe," Apple brings a quiet yet groundbreaking shift in how virtual storage is handled on Macs.
ASIF, which is its brand new disk image format, is poised to make encrypted storage and virtualization on Apple silicon machines significantly faster and more efficient.
With ASIF, Apple introduces a format capable of read and write speeds that nearly match native SSD performance.
In early benchmarks, a 100 GB ASIF image on a MacBook Pro M3 Pro hit 5.8 GB/s read and 6.6 GB/s write speeds on unencrypted APFS volumes. Even with encryption enabled, speeds held firm at 4.8 GB/s read and 4.6 GB/s write—a massive leap over previous formats. On a Mac mini M4 Pro, write speeds reportedly hit 8.3 GB/s.
ASIF lets files transfer more efficiently between hosts or disks, "because their intrinsic structure doesn’t depend on the host file system’s capabilities," according to a dedicated page on Apple’s developer website.
What makes ASIF different is the way it works.
ASIF images behave like sparse files in APFS—they only grow to hold actual data, meaning a 100 GB container might start at under 1 GB and remain small even after heavy usage. Even after extended use and mounting additional volumes, that size typically remains under 3.2 GB when empty.
They also consolidate all data into a single file, avoiding the complexity of sparse bundles, and are file-system-agnostic, making them portable and efficient across environments
Virtualization is where ASIF really shines.
The benefits for virtualization are especially notable. Apple now recommends virtual machines move away from older RAW (UDIF read-write) disk images in favor of ASIF. Not only does ASIF bring a performance boost, but it also offers easier portability when transferring VM files between hosts and drives.
It’s a promising new standard—especially considering ASIF performs better than traditional sparse bundles, which were previously favored for speed.
There are a few caveats.
Initially, ASIF image creation is only limited to macOS 26 “Tahoe”. Users also must rely on Disk Utility or the diskutil command-line tool, with no current support in hdiutil or previous macOS versions. What's more, Apple hasn’t confirmed backward compatibility, so users managing mixed-version environments should proceed with caution.
Developers and third-party tool creators are already responding. Apps like DropDMG by C-Command are expected to adopt ASIF support soon, but broader API access and documentation will be crucial for widespread integration.
Long story short, ASIF is fast, elegant, and forward-thinking—exactly the kind of quiet innovation Apple likes to slip into major OS updates.
For developers, power users, and virtual machine tinkerers, it could become the new gold standard in macOS storage.