Adrien here from the LogDevice team. A copy set is the list of storage node ids that the sequencer chose as recipients for a record. This piece of metadata is

Adrien here from the LogDevice team. A copy set is the list of storage node ids that the sequencer chose as recipients for a record. This piece of metadata is stored on storage nodes alongside record payloads in an index that maps the sequence number to copy set mapping. Storage nodes consult the index to filter out payloads they shall not send to readers. The filtering logic consists in sending the copy if the storage node sees itself as the first recipient in the copy set after it's been shuffled using the client id as a seed. This results in readers receiving exactly one copy of the payload.