In macOS 26 Tahoe, Apple has introduced a new disk image format known as the Apple Sparse Image Format (ASIF), designed to deliver read and write speeds that closely approximate those of native disks.
The primary objective behind the development of the ASIF format is to address performance constraints encountered when running virtual machines on Apple silicon. Its structural design is inherently independent of the host file system’s capabilities, and the actual disk space it occupies on the host depends solely on the volume of data stored within it.
For example, creating a 100GB virtual disk using the ASIF format does not result in the immediate allocation of 100GB of physical storage. If the virtual machine only generates 50GB of data, the ASIF image will correspondingly consume just 50GB of physical space.
In essence, files stored within an ASIF image are themselves sparse files—a type of file that allows for more efficient use of the file system when large portions remain empty. This characteristic is precisely why Apple has named the format “sparse image.”
Apple recommends users migrate their virtual machine storage images from the RAW format to ASIF to enhance file transfer performance between host and disk. However, implementation currently requires developer assistance, as ASIF support has not yet been integrated into virtual machine platforms and must be manually created via the diskutil command-line utility.
While performance may vary depending on the specific hardware, overall benchmarks indicate that ASIF provides significantly superior read/write speeds compared to traditional disk image formats.
