Affects Version/s: None
Fix Version/s: 2.6.0
Here is the scenario.
1. Normally before NN transitions a DN to decommissioned state, enough replicas have been copied to other "in service" DNs. However, in some rare situations, the cluster got into a state where a DN is in decommissioned state and a block's only replica is on that DN. In such state, the number of replication reported by fsck is 1; the block just stays in under replicated state; applications can still read the data, given decommissioned node can served read traffic.
This can happen in some error situations such DN failure or NN failover. For example
a) a block's only replica is node A temporarily.
b) Start decommission process on node A.
c) When node A is in "decommission-in-progress" state, node A crashed. NN will mark node A as dead.
d) After node A rejoins the cluster, NN will mark node A as decommissioned.
2. In theory, NN should take care of under replicated blocks. But it doesn't for this special case where the only replica is on decommissioned node. That is because NN has the policy of "decommissioned node can't be picked the source node for replication".
3. Given NN marks the node as decommissioned, admins will shutdown the datanode. Under replicated blocks turn into missing blocks.
4. The workaround is to recommission the node so that NN can start the replication from the node.