node in the figure represents a data node (ChunkServer). A CopySet is the basic unit of data replication in CurveBS. A CopySet contains multiple chunks. Copysets in CurveBS are created in advance. 1. Reduce metadata. If the replication group membership is saved for each chunk, the number of metadata will be large. 2. Example Reduce the number of replication groups. The size of chunk is such as 16MB by default. If replication groups are created based on chunk. The number of replication groups is large and the communication traffic between replication groups is heavy. Once a CopySet