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 immediately and adds no memory.

Copy-on-write

The clone and source share storage while 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 you barely alter stays near-free; one you rewrite fully costs as much as any file.

Usage

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

Uses

• 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.