June 28, 2026
Sparse Wars: Apple Strikes Back
Dissecting Apple's Sparse Image Format (ASIF)
Apple made a new file format and the internet instantly asked: why, exactly
TLDR: Apple introduced ASIF, a new space-saving disk file for virtual machines in macOS Tahoe, and one developer dug through it to figure out how it works. The community’s main response was equal parts skepticism and sarcasm: why make a new format at all, and why is Apple still so weird about simple file copying?
Apple quietly introduced a new way to store virtual computer disks in macOS Tahoe, and one curious developer responded by doing what the internet does best: tearing it apart byte by byte just to see how it works. On paper, this new format, ASIF, is meant to save space by only storing the parts of a big disk image that are actually used. In reality, the comments quickly turned into a mini courtroom drama over whether Apple built something useful or just reinvented a wheel that was already rolling fine.
The loudest reaction came from readers asking the brutally simple question: what does this do that older formats don’t? One commenter basically summed up the skeptical camp with a shrug and a side-eye at Apple, wondering why anyone needs this when other options already exist. That same comment swerved into a wildly relatable rant about how dragging an app out of a disk image and into Applications somehow still feels mysteriously slow, which gave the whole thread a nice "old man yells at installer" energy.
Then came the nerd-on-nerd fashion police. Another commenter zeroed in not on Apple’s format, but on the author’s parsing method, joking that using C-style code inside a Python script was "definitely a choice." In other words: even when the topic is a file format, the real show is still the comment section. Between reverse-engineering excitement, Apple skepticism, and programming-language snark, the community delivered exactly what it always does: questions, quips, and a tiny civil war over style.
Key Points
- •Apple introduced the ASIF disk image format in macOS 26 Tahoe at WWDC 2025.
- •The article describes ASIF as a sparse virtual disk format intended for virtual machines and compares it to VMDK, VHDX, and QCOW2.
- •The author created a 1 GiB ASIF test image with `diskutil`, attached it, and wrote a predictable block pattern to aid later verification.
- •The reverse-engineering process began with hexdump inspection, where the author identified file magic and likely big-endian integer fields.
- •A tentative ASIF header was defined and parsed with `dissect.cstruct`, revealing notable values including two `0x200` fields associated with 512-byte sector sizes.