Reflink copies

Copying a file within the same volume produces a block clone: an O(1) copy that shares the source's storage rather than duplicating it. The copy completes instantaneously and takes up no additional memory.

Copy-on-write

The clone and its source share storage while they remain identical. On the first write to either, only the modified region is given a private copy; the rest stays shared. The result is two independent files that cost only their differences. A clone that you barely alter stays near-free; and the one you rewrite fully costs as much as any file.

Use

No setting or command is required. Windows 11 24H2 and above uses block cloning for ordinary copy operations within a volume automatically, and applications that issue FSCTL_DUPLICATE_EXTENTS_TO_FILE benefit likewise. On macOS, Finder duplicates and cp -c clone in the same way. The clone and source must be on the same volume; copying across volumes is a normal full copy.

Examples and ideas

• Duplicate a dataset or project to experiment on, discard, and reclone from the original, with no copy time or memory doubling.
• Maintain several working copies of a tree that share their common data.
• Snapshot a large file before a risky change and discard on success.