lukenels / apfs-notes

APFS notes

APFS Notes

• Some notes and sample programs I wrote when playing around with APFS
• make will build three programs, modify, clone, and swap
  • modify changes the first 26 bytes of a file
  • clone uses the new COW copy API
  • swap atomically swaps two files
    • clone and swap won't work on non-APFS Volumes
• (Needs macOS >=10.12)
• alias apfs="diskutil apfs -IHaveBeenWarnedThatAPFSIsPreReleaseAndThatIMayLoseData" is convenient to have

APFS Stuff

APFS Container

• Imports APFS Physical stores
• Exports APFS Volumes
• Named by a particular physical store
  • Called the container reference disk
• (Only supports 1:1 mapping between physical stores and volumes for now)

Managing disks

• diskutil apfs -IHaveBeenWarnedThatAPFSIsPreReleaseAndThatIMayLoseData
• Aliased by apfs on my laptop

Create container

• apfs createContainer diskXsY
• apfs list shows global APFS state
• One container, one physical store

Create Volume

• apfs addVolume diskXsY APFS MyVolume
  • Space-sharing by default
  • Space-sharing volumes will share storage of underlying container
  • May result in surprising out-of-space errors
• apfs addVolume diskXsY APFS MyFixedVolume 10g
  • Fixed-size volume
  • Can't grow beyond original size
  • Guaranteed to have as much space as originally allocated

Play around with features

• dd if=/dev/random of=bigfile bs=1m count=512
• clone bigfile bigfile.2
  • du -ch . reports 1GB of data, apfs list reports ~512M of usage!
• for i in $(seq 500); do clone bigfile "bigfile.tmp.${i}"; done;
  • 251G vs ~512M!!
• Change first block
  • modify bigfile
  • Space doesn't increase much, most blocks unchanged
• Prepend to file
  • vim bigfile, insert character
  • Space increases a ton, all blocks after edit have been modified
• New atomic swap operation
  • swap bigfile bigfile.tmp.2
  • Claimed to be atomic by Apple

Implementation

• Copy-on-write
  • Clone operation creates file with references to old blocks
  • Only create copy of a block when it's modified
  • Saves space when performing lots of copies
  • Very different from hard link
    • Modifying hard-linked file changes original
    • Modifying COWed file creates copy, original stays same

Other things

• Snapshotting and Revisioning?
  • Doesn't seem to be available yet
  • No mention of either in man pages
• Encryption
  • Lots of encryption / privacy features
  • Fast erase: Give eff option to apfs addVolume
    • Not really encryption
    • How might it be implemented?
    • Doesn't work yet as far as I can tell
  • Per volume, per file, encryption

Other Resources

• Apple Documentation
• WWDC Presentation
• APFS on Wikipedia
• man diskutil
• man hdiutil
• man copyfile
• man renamex_ap