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APFS clones and snapshots: the kernel calls that make them work
clonefile, fclonefileat, fs_snapshot — three syscalls that let you copy 50 GB in 50 milliseconds. Here's what happens under each one, and what doesn't get copied.
apfsfilesystemclonefilesnapshots
Inside APFS: copy-on-write, snapshots, and the sealed system volume
Apple File System, the format under every modern Mac: how it lays out blocks, how it gets snapshots almost for free, and why your /System is read-only at the cryptographic level.
apfsfilesystemkernelxnu