\nApple Core Storage debuted with macOS 10.7 Lion and remained a key part of the storage stack through macOS 10.13 High Sierra. Think of it as Apple\u2019s version of a logical volume manager (LVM)\u2014similar in spirit to Linux LVM. Instead of writing a file system directly onto a fixed partition, Core Storage inserts a virtualization layer between the physical partition map and the file system you mount. That layer allows macOS to manage data more flexibly than with partitions alone.<\/p>\n
Traditionally, a partition map carves a disk into discrete regions (\u201cpartitions\u201d), and each partition gets its own file system. It\u2019s simple and predictable\u2014but rigid. A logical volume manager, by contrast, lets the operating system allocate space dynamically and even build a single logical volume out of multiple physical devices.<\/p>\n
Core Storage originally shipped to enable two headline features:<\/p>\n
\n- FileVault 2<\/a><\/b> \u2013 true full\u2011disk, block\u2011level encryption for your Mac.<\/li>\n
- Fusion Drive<\/b> \u2013 a tiered setup that blends an SSD\u2019s speed with an HDD\u2019s capacity.<\/li>\n<\/ul>\n
How Core Storage Organizes Data<\/b><\/h4>\n
Core Storage uses four object types, each identified by a UUID. The names are Apple\u2011specific, but the concepts will feel familiar if you know LVM.<\/p>\n
\n- Physical Volume (PV)<\/b>: The raw building block. This is usually a real disk (HDD\/SSD), but it can also be a disk image or a member of an AppleRAID set. To participate, the device is typically partitioned with GUID Partition Table (GPT) and has its own GUID. Each PV stores small bits of metadata about the group it belongs to.<\/li>\n
- Logical Volume Group (LVG)<\/b>: A storage pool formed by one or more PVs. The LVG is where logical volumes live and from which they draw capacity. In many setups, you\u2019ll see a single logical volume that spans the sum of all PVs.<\/li>\n
- Logical Volume Family (LVF)<\/b>: An Apple\u2011specific container for shared properties\u2014most notably encryption settings\u2014that apply to one or more logical volumes inside the LVG.<\/li>\n
- Logical Volume (LV)<\/b>: The virtual device that receives a file system (historically HFS+) and mounts in Finder. From the user\u2019s perspective, the LV looks and behaves like any other volume.<\/li>\n<\/ul>\n
Core Storage Data Organization: Layout, Metadata & Behavior<\/b><\/h4>\nFileVault 2 (block\u2011level full\u2011disk encryption)<\/b><\/h5>\n
Prior to Lion, FileVault encrypted user data within files. Core Storage moved encryption to the block layer. When you unlock an encrypted disk, Core Storage exposes a decrypted logical volume you can mount. The keys and encryption context live in Core Storage metadata, which is why losing that metadata (or the relevant key files) can make decryption impossible.<\/p>\n
A simplified example you might see in Terminal:<\/p>\n
\n\/dev\/disk0<\/code> \u2014 the physical device<\/li>\n\/dev\/disk0s3<\/code> \u2014 the Core Storage Physical Volume<\/b> that stores encrypted content and is a member of the LVG<\/li>\n\/dev\/disk1<\/code> \u2014 the Logical Volume<\/b> presented after you unlock and decrypt<\/li>\n<\/ul>\nFusion Drive (SSD + HDD, presented as one)<\/b><\/h5>\n
A typical Fusion setup looks like:<\/p>\n
\ndev\/disk0<\/code> \u2014 the SSD (a PV in the LVG)<\/li>\ndev\/disk1<\/code> \u2014 the HDD (another PV in the LVG)<\/li>\ndev\/disk2<\/code> \u2014 the LV spanning both devices<\/li>\n<\/ul>\nThe SSD is marked primary, so frequently accessed data gravitates to flash in roughly 128 KB chunks; colder data flows back to the HDD. Under the hood, Core Storage uses internal routines (historically surfaced as names like RdChunkCS<\/code>, WrChunkCS<\/code>, WrBgMigCS<\/code>, and
![\"Fusion](\"\/help\/wp-content\/uploads\/2018\/08\/fusion-drive-disk-drill-mac.jpg\")
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