Dissecting Apple’s Sparse Image Format (ASIF)
ASIF takes a lot of inspiration from existing virtual disk formats. Practically, that means it’s another sparse virtual disk format, and functions very similar to sparse VMDK, VHDX or QCOW2 files (for the uninitiated, it allow you to store a large disk, or file, in a smaller, “sparse” manner).
Shortly before the release of macOS Tahoe (late 2025), I thought it’d be a fun exercise to try and write a parser for ASIF files.
Previously:
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I mean, would it have killed them to just use QCOW2 themselves? Why do we need yet another disk image format that adds nothing substantially new? And why not improve on their existing "sparse" disk image format?
Anyway. In these enlightened times of copy-on-write filesystems, there's not a compelling reason not to simply use "flat" images with sparse regions. The resulting files are logically coherent and portable, and can be examined and used by many more tools. Just go with that.
