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Map · The on-disk format

APFS

Apple File System: copy-on-write, snapshots, cloning, sealed system volumes — the format under every modern Mac.

apple-oss-distributions/xnubsd/vfs/vfs_syscalls.cStart reading the kernel source here.View on GitHub(line —)
APFS container layoutOne APFS container per partition holds multiple volumes that share a single free-space pool. The system volume is read-only and cryptographically sealed; the data volume is writable, joined to the system volume by firmlinks.PHYSICAL DISKSSD (single block device)APFS containersingle shared free-space poolMacintosh HDsystem · sealed · read-only⛨ SSV root hashMacintosh HD - Datauser data · read/writePrebootboot policyRecoveryrecovery OSVMswap filesfirmlink — joins read-only system + writable data into one logical /df reports the same free space for every volume — because there is only one pool.Every write goes to a fresh block. Snapshots are free — they just pin old blocks against GC.