Mac storage glossary.

The words behind your disk space, explained in plain language.

The file system on every modern Mac startup disk, designed for SSDs, with copy-on-write, clones, snapshots and shared free space.

APFS replaced HFS+ on Mac startup disks with macOS High Sierra in 2017, and every startup disk has used it since macOS Mojave. Because it writes changes to new blocks instead of overwriting old ones, it can make instant clones and snapshots, which is also why the same data can be counted once by one tool and twice by another. Run diskutil apfs list in Terminal to see your containers, volumes and their roles.

Also called: Apple File System

APFS container

A single storage pool on a disk that holds one or more APFS volumes, all drawing on the same free space.

Unlike classic partitions, volumes in a container have no fixed size: each grows and shrinks as needed, and free space belongs to the container as a whole. That is why the volumes on your startup disk all report the same available space, and why space used by a hidden volume such as VM or Preboot reduces what is left for your files. In Disk Utility, choose View › Show All Devices to see the container, or run diskutil apfs list.

Also called: APFS partition, container disk

APFS volume roles

Labels that tell macOS what each volume in the startup container is for: System, Data, Preboot, Recovery or VM.

Since macOS Catalina, the startup disk is split into a read-only System volume that holds macOS and a Data volume that holds your apps, accounts and files; Finder shows them together as one disk, usually named Macintosh HD. Preboot holds files needed to start up, Recovery holds macOS Recovery, and VM holds swap. diskutil apfs list shows each volume’s role and the space it consumes; only the Data volume contains things you manage yourself.

Also called: System volume, Data volume, VM volume

APFS clone

A copy of a file on the same APFS volume that shares the original’s data blocks, so it uses almost no extra space until edited.

Finder’s Duplicate command and cp -c create clones; when either copy changes, APFS writes only the modified blocks to new storage. Each clone still shows the full file size in Finder, so a folder of clones can look much larger than the space it really uses, and deleting one clone frees little or nothing. Clones exist only within a single volume: copying to another disk writes all the data again.

Also called: clone file, copy-on-write copy

APFS snapshot

A read-only, point-in-time image of an APFS volume that keeps every block it references from being freed while it exists.

Creating a snapshot is instant and initially uses no extra space, but as files are changed or deleted afterwards, their old blocks stay pinned by the snapshot. This is why deleting a large file sometimes frees nothing until the snapshot holding it is removed. macOS uses snapshots for Time Machine, for macOS updates and to start up from the sealed system volume; diskutil apfs listSnapshots /System/Volumes/Data lists those on your Data volume.

Also called: volume snapshot

Local snapshot

An APFS snapshot of your startup disk that Time Machine keeps on the Mac itself, so files can be restored without the backup disk.

With Time Machine set up, macOS saves a snapshot about every hour and keeps each one for 24 hours, plus a snapshot of the last successful backup until space is needed; it also saves one before each macOS update. Snapshots are stored only when the disk has plenty of free space, macOS counts their space as available, and it deletes them automatically as they age or when room is needed. tmutil listlocalsnapshots / lists them, and tmutil deletelocalsnapshots <date> deletes one permanently.

Also called: Time Machine local snapshot, com.apple.TimeMachine snapshot

Purgeable space

Space in use that macOS can reclaim on demand, such as local snapshots, some caches and iCloud files already stored in the cloud.

Finder and System Settings count purgeable space as available, while df and many other tools count it as used, which is the main reason their figures disagree. macOS releases it automatically, often gradually, when an app or an update needs room, so you normally don’t need to do anything. If an installer reports too little space despite a large available figure, the purgeable part may not have been released yet; Get Info on the startup disk shows how much of the available space is purgeable.

Also called: Purgeable, purgeable storage

System Data

The category in macOS Storage settings for files that don’t fit its other categories, such as caches, logs, snapshots and swap.

Shown in System Settings › General › Storage and called Other in older versions of macOS, it typically includes caches, logs, local Time Machine snapshots, swap, virtual machine and container disk images, app support data and hidden files. Its size varies widely from one Mac to another and often shrinks on its own as macOS purges snapshots and caches. To find out what it contains, look in ~/Library and run tmutil listlocalsnapshots / rather than deleting anything under /System.

Also called: Other storage, System Data storage

Signed System Volume (SSV)

Since macOS Big Sur, the read-only volume containing macOS, cryptographically sealed so that any change to system files can be detected.

Every file on the System volume is covered by a tree of hashes whose root, the seal, is signed by Apple and verified when the Mac starts up, and the Mac actually runs from a sealed snapshot of that volume. Updates prepare and seal a new snapshot instead of modifying files in place. Nothing on the System volume can be removed to free space; its size, typically more than ten gigabytes, appears as macOS in Storage settings.

Also called: sealed system volume, SSV

Firmlink

A special two-way link, introduced in macOS Catalina, that makes folders on the Data volume appear inside the read-only System volume.

Thanks to firmlinks, paths such as /Users, /Applications and /Library look like part of one disk although they live on the Data volume, which is also mounted at /System/Volumes/Data. The full list is in /usr/share/firmlinks. A tool that scans both / and /System/Volumes/Data without recognising firmlinks can count the same files twice.

Also called: firm link

Preboot volume

A hidden APFS volume holding files your Mac needs before macOS itself starts, including boot and FileVault unlock data.

It is mounted at /System/Volumes/Preboot and managed entirely by macOS. On recent versions of macOS it also holds system components delivered as separately signed images (cryptexes), so it typically uses several gigabytes and can exceed ten, more when several macOS installations share the container. Don’t delete anything in it by hand: a damaged Preboot volume can stop the Mac from starting up.

Also called: Preboot

Recovery volume

A hidden APFS volume containing macOS Recovery, used to reinstall macOS, restore from a Time Machine backup or repair the disk.

It typically uses one to a few gigabytes and is updated along with macOS; Apple silicon Macs also keep a fallback copy of recoveryOS in a separate hidden container. It cannot and should not be removed to free space. To start up in Recovery, press and hold the power button on a Mac with Apple silicon, or hold Command-R at startup on an Intel Mac.

Also called: macOS Recovery, recoveryOS

Swap

Disk space macOS uses as overflow memory, writing memory contents to disk when apps need more than the installed RAM can hold.

macOS first compresses memory, then writes encrypted swap files to the VM volume (/System/Volumes/VM on recent versions), adding files as needed. Swap can grow to several gigabytes during heavy use and is released when you restart. sysctl vm.swapusage shows the current total; never delete swap files by hand.

Also called: swap file, swapfile, virtual memory

Sleep image

A file in which an Intel Mac saves the contents of memory so it can resume after deep sleep or if the battery runs out.

Its size is tied to the amount of installed memory, and pmset -g shows its location (hibernatefile) and the current hibernatemode. Apple silicon Macs handle sleep differently and usually have no sleep image; macOS 27 runs only on Apple silicon. If deleted, it is recreated, and changing hibernation settings to save space is not recommended.

Also called: sleepimage, hibernation file

Sparse file

A file whose empty regions are not stored on disk, so its logical size can be far larger than the space it occupies.

APFS supports sparse files: regions that were never written take no blocks. Docker Desktop’s Docker.raw and many virtual machine disks work this way and can show their maximum size while using much less. Compare ls -lh (logical size) with du -h (space allocated), and copy such files with tools that preserve sparseness, or the copy may take the full size.

Also called: file with holes

Logical vs physical size

Logical size is how many bytes a file contains; physical size is how much disk space it actually occupies.

The two differ because of block rounding, transparent compression, sparse files and clones: a sparse or compressed file can use far less than its logical size, while thousands of tiny files each occupy at least one block. Finder’s Get Info shows both figures (bytes and on disk), ls -l reports logical size and du -sh reports allocated space. Neither reveals blocks shared by clones or hard links, so adding up sizes can overstate what deleting files would free.

Also called: size on disk, allocated size, apparent size

Decimal vs binary units (GB vs GiB)

A gigabyte (GB) is 1,000,000,000 bytes; a gibibyte (GiB) is 1,073,741,824 bytes, about 7% more.

Since Mac OS X Snow Leopard (10.6), Finder and System Settings use decimal units, as drive makers do, so a 1 TB SSD (1,000,000,000,000 bytes) shows as about 1 TB, which is about 931 GiB in binary units. Some command-line tools use binary units: on macOS, df -h and du -h count in powers of 1,024 (df labels them Gi), while df -H uses powers of 1,000. When two figures differ by about 7% for gigabytes or about 10% for terabytes, units are the likely cause.

Also called: GB vs GiB, gigabyte vs gibibyte, base 10 vs base 2

Hard link

An additional name for the same file data: several paths point to one file, and its space is used only once.

The data is freed only when the last link is deleted, so removing one path of a hard-linked file frees nothing. Tools that add up every path without checking can count the same data several times; ls -l shows the link count and stat -f %l <file> prints it. APFS supports hard links to files but not to folders, and package managers such as pnpm use file hard links to share dependencies between projects.

Also called: hardlink

Symbolic link

A tiny file that points to another path; opening it opens the target, but it holds none of the target’s data.

Created with ln -s, a symlink appears with an arrow in ls -l output; deleting it leaves the target untouched, and deleting the target leaves a broken link. du does not follow symlinks by default, so moving a large folder to another disk and leaving a symlink in its place is a common way to relocate data. Finder aliases look similar but are a separate macOS mechanism that can keep track of a moved target.

Also called: symlink, soft link

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