I'm on the LogDevice team at Facebook. The "copy set" is the subset of server machines that a given record is stored on. For example, if you have a cluster with 10 machines numbered 0 through 9, then the copy set for record 1 might be 3, 6, 8; for record 2 might be 0, 2, 5, etc. Copy set can change with each record. The node set (the list of 10 machines) is fixed for the length of an epoch.