Reflink

Reflink is a type of shallow copy of file data that shares the blocks but otherwise the files are independent and any change to the file will not affect the other. This builds on the underlying COW mechanism. A reflink will effectively create only a separate metadata pointing to the shared blocks which is typically much faster than a deep copy of all blocks.

The reflink is typically meant for whole files but a partial file range can be also copied, though there are no ready-made tools for that.

cp --reflink=always source target

There are some constraints:

• cross-filesystem reflink is not possible, there’s nothing in common between so the block sharing can’t work

• reflink crossing two mount points of the same filesystem support depends on kernel version:

  • until 5.17 it’s not supported and fails with “Cross device link”, can be worked around by performing the operation on the toplevel subvolume

  • works since 5.18

• reflink requires source and target file that have the same status regarding NOCOW and checksums, for example if the source file is NOCOW (once created with the chattr +C attribute) then the above command won’t work unless the target file is pre-created with the +C attribute as well, or the NOCOW attribute is inherited from the parent directory (chattr +C on the directory) or if the whole filesystem is mounted with -o nodatacow that would create the NOCOW files by default