macOS Tahoe brings a new disk image format

Summary

Disk images have been valuable tools marred by poor performance. In the wrong circumstances, an encrypted sparse image (UDSP) stored on the blazingly fast internal SSD of an Apple silicon Mac may write files no faster than 100 MB/s, typical for a cheap hard drive. One of the important new features introduced in macOS 26 Tahoe is a new disk image format that can achieve near-native speeds: ASIF, documented here. This has been detailed as a major improvement in lightweight virtualisation, where it promises to overcome the most significant performance limitation of VMs running on Apple silicon Macs. However, ASIF disk images are available for general use, and even work in macOS Sequoia. This article shows what they can do. Apple provides few technical details, other than stating that the intrinsic structure of ASIF disk images doesn’t depend on the host file system’s capabilities, and their size on the host depends on the size of the data stored in the disk. In other words, they’re a sparse file in APFS, and are flagged as such. Make an ASIF disk image Currently, there are only two ways to create one of these new disk images, either in Tahoe’s Disk Utility, or using its diskutil command tool, as in diskutil image create blank --format ASIF --size 100G --volumeName myVolume imagePath to create an ASIF image with a maximum capacity of 100 GB with a single APFS volume named myVolume with the path and name imagePath. You can also use a from option to convert an existing disk image to ASIF format. These are only good for Tahoe, as there’s no support for their creation in Sequoia 15.5 or earlier. Neither is there any access documented for the hdiutil command tool, more normally used to work with disk images, although its general commands should work fine with ASIFs. Resulting disk images have a UTI type of com.apple.disk-image-sparse, in contrast to RAW (UDIF read-write) images of type com.apple.disk-image-udif, which can be used to distinguish them. Economy When first creat...