I add new node to exist cluster in cassandra and waiting to node join to cluster. after that, I update replication factor and repair each affected node according to Updating the replication factor. but why repairing node takes a long time?
Repair process depends on the amount of data that you have. Repairing 100GB of data usually (depending on your instances or servers) and the load on your cluster takes around 1 hour, this is some very vague rule of thumb. If you have large amounts of data please take into account that it may take hours before the repair is actually finished. It also depends on the cassandra version that you are using. Some versions of cassandra simply hang the repair process, please check system.log for more information. If you notice that repair failed, you might want to consider upgrading cassandra.