subvolumes

Subvolumes are a feature of certain copy-on-write filesystems, most notably Btrfs. A subvolume is a separate, independently addressable subtree inside a single Btrfs filesystem. It provides a way to organize data into logical units that can be mounted at different paths and managed independently, without requiring separate block devices.

A subvolume exists within a Btrfs volume as a distinct root tree. It shares the underlying storage with other subvolumes but has its own root directory and can be mounted separately. Subvolumes are created within an existing Btrfs filesystem using commands such as btrfs subvolume create, and they can be listed with btrfs subvolume list. When mounting, a subvolume is selected with the mount option subvol=NAME, allowing multiple subvolumes to appear as separate roots at different mount points.

Snapshots are closely associated with subvolumes. A snapshot is a point-in-time copy of a subvolume that shares data blocks with its source (thanks to copy-on-write), so snapshots are typically lightweight to create. Snapshots can be read-only or writable, depending on the system’s needs and the available tooling.

Use cases for subvolumes include organizing a system into separate trees for the root filesystem, home directories, or variable data; enabling targeted backups and rollbacks; and applying per-subvolume management such as quotas or different mount options. Subvolumes can be nested and renamed within a filesystem, and deleting a subvolume removes only that subtree, reclaiming blocks that are no longer referenced.

In summary, subvolumes provide modular, mountable, and snapshot-friendly partitions within a single Btrfs filesystem, improving organization, backup strategies, and recovery options without extra devices.