I thought decommissioning all of them would automatically copy data to commissioned nodes. That did not happen though. I currently have 4 DN's in the cluster. I need to remove all 4 of them add 3 new ones instead. I need to maintain the data ofcourse.
Decommision 1 datanode at a time so that the data get replicated to another node and then go for the next. Make sure there is no underreplicated data in namenodeUI before you decommision another datanode.
Please accept this as sollution if it resolve your issue