The actual act of snapshotting doesn’t do a whole lot. I believe ZFS uses CoW for that purpose. If you modify a file after a snapshot, the file (or block) will

The actual act of snapshotting doesn’t do a whole lot. I believe ZFS uses CoW for that purpose. If you modify a file after a snapshot, the file (or block) will be copied first. Normally, with CoW, the original would then be marked as free space available for writing, but if a snapshot exists, it will be preserved. (This is a general concept; I don’t know the nuances of ZFS’ approach.)