This was observed while doing HBase WAL recovery. HBase uses append to write to its write ahead log. So initially the pipeline is setup as
DN1 --> DN2 --> DN3
This WAL needs to be read when DN1 fails since it houses the HBase regionserver for the WAL.
HBase first recovers the lease on the WAL file. During recovery, we choose DN1 as the primary DN to do the recovery even though DN1 has failed and is not heartbeating any more.
Avoiding the stale DN1 would speed up recovery and reduce hbase MTTR. There are two options.
a) Ride on HDFS 3703 and if stale node detection is turned on, we do not choose stale datanodes (typically not heart beated for 20-30 seconds) as primary DN(s)
b) We sort the replicas in order of last heart beat and always pick the ones which gave the most recent heart beat
Going to the dead datanode increases lease + block recovery since the block goes into UNDER_RECOVERY state even though no one is recovering it actively. Please let me know if this makes sense. If yes, whether we should move forward with a) or b).