When choosing a source node to replicate a block from, a decommissioning node is favored. The reason for the favoritism is that decommissioning nodes aren't servicing any writes so in-theory they are less loaded.
However, the same selection algorithm also tries to make sure it doesn't get "stuck" on any particular node:
Unfortunately, the decommissioning check is prior to this randomness so the algorithm can get stuck trying to replicate from a decommissioning node. We've seen this in practice where a decommissioning datanode was failing to replicate a block for many days, when other viable replicas of the block were available.
Given that we limit the number of streams we'll assign to a given node (default soft limit of 2, hard limit of 4), It doesn't seem like favoring a decommissioning node has significant benefit. i.e. when there is significant replication work to do, we'll quickly hit the stream limit of the decommissioning nodes and use other nodes in the cluster anyway; when there isn't significant replication work then in theory we've got plenty of replication bandwidth available so choosing a decommissioning node isn't much of a win.
I see two choices:
1) Change the algorithm to still favor decommissioning nodes but with some level of randomness that will avoid always selecting the decommissioning node
2) Remove the favoritism for decommissioning nodes
I prefer #2. It simplifies the algorithm, and given the other throttles we have in place, I'm not sure there is a significant benefit to selecting decommissioning nodes.