reflink copy
also: CoW copy, copy-on-write copy
A copy operation that creates a new file sharing the same data blocks as the original until one is modified, reducing storage space and improving speed. This copy-on-write approach avoids duplicating data immediately.
Reflink copy is a fast, space-efficient file copying method supported by modern filesystems like Btrfs and XFS. Instead of immediately duplicating all data blocks, the new file initially shares the same underlying blocks as the source. When either file is modified, the filesystem automatically creates a separate copy of the affected blocks—a process called copy-on-write (CoW).
This is particularly useful for virtual machine images, database snapshots, and large file backups where you want independent copies but don't need immediate duplication. For example, cp --reflink source.img snapshot.img creates a new VM image that shares blocks with the original until one is written to.
Reflink copies are much faster and use less disk space than traditional copies initially, but transparent CoW ensures data independence. If the filesystem doesn't support reflink, the operation falls back to a standard copy, making it safe to use universally.