thanks for your reply.
The new node in different DC is added as same DC, so i mean I use quorum or all for three replicate not local quorum.
does the long latency will block coordinate and affect the performance？
If you use local quorum it won't impact read and write latencies but there will be delay in across DC replication; which may cause some application issues if it is active active. But 2ms is probably negligible to see any effect.
I am adding a new node to cassandra，but the new node is at different DC，the ping latency between them is 2 ms.
I am wondering does the long latency would affect cluster's performance or thoughput？