reflink
also: CoW copy, copy-on-write clone, reference link
A filesystem feature that creates instant, copy-on-write clones of files or ranges of data without duplicating the actual blocks on disk, saving space and time.
Reflink (reference link) is a modern filesystem operation that creates a new file or block range pointing to the same underlying data as the source, without immediately copying bytes. When either the original or the clone is modified, only the changed blocks are duplicated—this is called copy-on-write (CoW).
Unlike traditional cp, which duplicates all blocks immediately and takes time proportional to file size, reflink is near-instantaneous and uses zero extra space until changes occur. This is particularly valuable for large files, VM images, and backups.
Example: cp --reflink source.img clone.img creates an instant clone. If the original is 10 GB but you only modify 100 MB, only that 100 MB is actually duplicated on disk.
Reflink support varies by filesystem: Btrfs, XFS, and APFS support it natively, while ext4 does not. The cp, dd, and other tools can use it with the --reflink flag.