WWDC: What is Apple Sparse Image Format (ASIF)?

Apple stealthily introduced Apple Sparse Image Format (ASIF), a new sparse disk image format for Apple Silicon, at WWDC; among other features, it might also help Macs remain the best PCs on which to run Windows.

That somewhat counter-intuitive claim is because the new format dramatically improves the efficiency with which Apple Silicon Macs run virtual machines (VMs) by boosting read/write performance. ASIF also makes significant improvements to how Macs handle storage for VMs, and is likely to support third-party virtualization tools once it ships.

What is Apple Sparse Image Format (ASIF)?

Set to appear with macOS Tahoe later this year, 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 Apple’s developer website. “The size the ASIF file takes on the file system is proportional to the actual data stored in the disk image.”

[ Related: Apple WWDC 2025: News and analysis ]

ASIF replaces the currently used format, which occupies the same amount of disk space as the allocated portion of the disk. What this means is that when you allocate 10GB of disk space to a VM you immediately sacrifice 10GB of space, no matter how much data the virtual machine contains; with ASIF, the volume will only occupy as much space as it contains. In other words, you can allocate a large quantity of disk space to enable optimal VM performance but sacrifice only as much actual space as the VM contains. 

“These space-efficient images can be created with the diskutil image command-line tool or the Disk Utility application and are suitable for various uses, including as a backing store for virtual machines storage via the Virtualization framework,” Apple explained.

The company says users should migrate their VM storage images from the existing RAW format to ASIF to benefit from the improved file transfer performance between the host Mac and the disk. 

Faster and highly performant

Eclectic Light was first out the gate with news about Apple’s new tech, publishing a first look alongside test results to show how much faster it is in use than standard sparse imaging technologies it is.

The test results show that ASIF gives Disk images on Apple Silicon devices near-native SSD speeds. That matters whenever you are moving data around, and is particularly important when running Linux or Windows in virtual machines. It means you should experience significant performance benefits, further reinforcing the Mac as the best platform for Windows.

Eclectic Light noted that in some cases an encrypted sparse image (UDSP) stored on the fast SSD of a current Mac might only write files at up to an unimpressive 100 MBps. That report comes with receipts, sharing extensive test data to show that even encrypted ASIF files read and write data far faster than the sparse file formats Macs use today. That’s will mean much more moving forward as on-device encryption becomes even more essential to personal data protection as government mandated back doors are identified and abused.

It’s not just about virtual machines. For general storage, it seems highly probable the new format will also improve the performance of FileVault. That’s because right now encrypted sparse images are used to secure a user’s home directory in FileVault, so better performance and storage management have implications there. (I also speculate that the new format might have implications in how Apple efficiently provides and encrypts future LLM services via Private Cloud Compute.)

Additional WWDC coverage:

WWDC: For developers, Apple’s tools get a lot better for AI

Apple’s AI Revolution: Insights from WWDC

WWDC 2025: What’s new for Apple and the enterprise

WWDC: What we know so far about Apple’s Liquid Glass UI

WWDC first look: How Apple is improving its ecosystem.

Apple WWDC 2025: News and analysis

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