block clone
also: reflink, copy-on-write, CoW, block reference
A filesystem operation that creates a new reference to existing data blocks without duplicating the actual data, reducing storage space and improving performance for identical content.
Block cloning (also called copy-on-write or reflink) is a filesystem technique where multiple files or snapshots can share the same underlying data blocks. Instead of copying data, the filesystem creates a new reference pointing to the existing blocks, saving disk space and I/O time.
When one of the cloned copies is modified, the filesystem creates separate blocks for that file (copy-on-write behavior), leaving other references intact. This is particularly useful for virtual machine images, backups, and large file operations.
Example: Creating a backup with cp --reflink file1 file2 on a Btrfs or XFS filesystem creates file2 pointing to file1's blocks. Both files initially share the same data, using nearly no extra space. When you edit file2, new blocks are allocated only for the changed portions.