CoW refers to how data is written to the disk. With CoW, data is never written back over the same block it came from, it’s always written to a new block, then the old block is unreserved after. In this way, data moves around on a CoW disk frequently. This attribute notably makes the filesystem very resilient against power loss, since data can’t be corrupted during a partial write. Only if the write is completed successfully will the old data be removed.