Definition
Plain language
A filesystem feature where copying a file doesn't actually copy the data — both names point at the same blocks until one is edited.
As stated in the literature
A copy-on-write file-copy primitive supported by XFS and other modern filesystems that shares underlying disk blocks by reference; central to DeltaBox's storage cost scaling with delta size rather than snapshot count.
Also called: reflinks
Why it matters: It makes per-step sandbox snapshots cheap enough that an agent can save state at every action, enabling true rewindable experimentation and tree-search.
For example, a 10-gigabyte virtual machine image can be "copied" instantly with a reflink because the new file just points at the same disk blocks until something changes.
Heard on the show
“… And underneath the whole thing they use XFS with reflinks turned on, which means when a file does get copied up from a read-only layer because somebody's …”Episode 068 — The OS Trick That Makes Tree Search Practical for Coding Agents