Subvolume

Subvolume is a term used in Btrfs, a copy-on-write filesystem for Linux, to describe a logical subtree within a single Btrfs volume. A subvolume is not a separate block device or a distinct filesystem; rather, it is an independent, mountable subtree that shares the storage resources of the containing volume. Subvolumes can be created, mounted, snapshotted, and managed separately, enabling flexible organization of data and configurations within one disk partition.

Creation and mounting: Subvolumes are created with the btrfs subvolume create command at a path inside the filesystem. They can be mounted directly by specifying the subvolume option on mount, for example mount -o subvol=@subvol /dev/sdXN /mnt. A Btrfs filesystem can contain multiple subvolumes, including a top-level subvolume that serves as a root for other subvolumes.

Snapshots and copy-on-write: Subvolumes support snapshots, which are lightweight, point-in-time views of their data. A snapshot is itself a subvolume; read-only snapshots can be created with -r. Because Btrfs uses copy-on-write, snapshots initially consume little additional space and only diverge as changes are made.

Quotas and management: Quotas can be applied to subvolumes using Btrfs quota and qgroup features, which require enabling quotas on the filesystem. This allows tracking and limiting space per subvolume or per group of subvolumes.

Limitations and concepts: Subvolumes share the same block device and pool of blocks; deleting one subvolume does not affect others except for freeing space when data is no longer referenced. They are a core organizational unit in Btrfs and are commonly used for system trees, user data separation, backups, and testing.