Are Containers, Volumes, or Partitions All the Same?

Learn the differences between these macOS file system components

Disk Utility and the APFS file system

Coyote Moon, Inc.

Containers, volumes, and partitions are elements of a computer's file management system. These are similar objects, but there are small yet significant differences. Here's a description of each of these objects and how these objects interact.

What Is a Volume?

A volume is a storage container in a file system that a computer can recognize. Common types of volumes include CDs, DVDs, SSDs, hard drives, and partitions of SSDs or hard drives.

Volume vs. Partition

A volume such as a hard drive can be divided into one or more partitions. Each partition takes up space on the hard drive. For example, in a 1 TB drive containing four 250 GB partitions, the first two partitions have standard Mac file systems, the third partition has a Windows file system, and the final one either has no formatting or contains a file system that the Mac doesn't recognize.

The Mac sees the two Mac partitions and the Windows partition (because the Mac can read Windows file systems), but it doesn't see the fourth partition. It's still a partition, but it's not a volume, because the Mac can't recognize any file system on it.

When a Mac recognizes a volume, it mounts the volume on the desktop so that you can access any data it contains.

Logical Volumes

A more abstract type of volume, known as a logical volume, isn't limited to a single physical drive. It can house as many partitions and physical drives as needed.

A logical volume allocates and manages space on one or more mass storage devices. It separates the operating system from the physical devices that make up the storage medium.

A basic example of this is RAID 1 (mirroring), in which multiple volumes appear to the OS as a single logical volume. Both hardware controllers and software can create RAID arrays. In both cases, the OS isn't aware of what's physically making up the logical volume. It could be one drive, two drives, or many drives. The number of drives making up the RAID 1 array can change over time, and the OS is never aware of these changes. All the OS ever sees is a single logical volume.

With a logical volume, not only is the physical device structure independent of the volume the OS sees, but a user can also manage it independently of the OS. This setup allows for very simple or very complex data storage systems.

In addition to RAID 1, the other common RAID systems make use of multiple volumes that appear to the OS as a single logical volume. But RAID arrays aren't the only storage system that uses a logical volume.

Logical Volume Manager (LVM)

Logical volumes contain partitions located on multiple physical storage devices. Logical Volume Managers (LVMs) make these systems easier to use. The LVM manages a storage array, allocates partitions, creates volumes, and controls how the volumes interact with each other.

Since Apple introduced OS X Lion, the Mac has had an LVM system known as core storage. It was first used to provide the full-disk encryption system used by Apple’s File Vault 2 system. Then, when OS X Mountain Lion was released, the core storage system gained the ability to manage a tiered storage system that Apple called a Fusion drive.

Containers

With the addition of the APFS (Apple File System) in macOS High Sierra, containers took on a new organizational space in the file system.

APFS is all about containers, a logical construct of space that contains one or more volumes. Multiple containers can exist, and each makes use of the APFS file system. Individual volumes within an APFS container must use the APFS file systems.​​

When all of the volumes within a container use the APFS file system, the volumes share the space available in the container. A volume that needs additional storage space uses free space in the container. Unlike partitions, the volume can use free space from anywhere. The space-sharing works even if the volumes aren't adjacent, which was necessary before APFS.