reflink

Reflink, short for reference link, is a feature used by some copy-on-write file systems to create a new file that shares the same data blocks as an existing file. The two files appear as separate entities, but initially they reference the same on-disk blocks. A write to either file triggers a copy-on-write operation, resulting in distinct data blocks for the modified file while the other retains its original content.

This capability is commonly available on copy-on-write file systems such as Btrfs and similar technologies. In practice, reflink allows a fast, space-efficient “copy” of a file, since no data is immediately duplicated. Users can create such a copy via high-level tools that support reflink, notably the cp command with the --reflink option (for example, cp --reflink=auto src dst). If the destination filesystem does not support reflink, the tool will usually fall back to a regular data copy.

Scope and limitations: Reflink copies are only possible within the same filesystem that implements the feature. They do not cross filesystem boundaries. Deleting one of the two files does not necessarily reclaim all data blocks until the last link is removed. The performance benefit includes reduced I/O and faster duplication for large files, but some backup and deduplication strategies may not treat reflinked copies as independent.

See also: Copy-on-write, cloning, lightweight copies, Btrfs.