When we decommission nodes across different racks, the decommission process becomes really slow at the end, hardly making any progress. The problem is some blocks are on 3 decomm-in-progress DNs and the way how replications are scheduled caused unnecessary delay. Here is the analysis.
When BlockManager schedules the replication work from neededReplication, it first needs to pick the source node for replication via chooseSourceDatanode. The core policies to pick the source node are:
1. Prefer decomm-in-progress node.
2. Only pick the nodes whose outstanding replication counts are below thresholds dfs.namenode.replication.max-streams or dfs.namenode.replication.max-streams-hard-limit, based on the replication priority.
When we decommission nodes,
1. All the decommission nodes' blocks will be added to neededReplication.
2. BM will pick X number of blocks from neededReplication in each iteration. X is based on cluster size and some configurable multiplier. So if the cluster has 2000 nodes, X will be around 4000.
3. Given these 4000 nodes are on the same decomm-in-progress node A, A end up being chosen as the source node of all these 4000 nodes. The reason the outstanding replication thresholds don't kick is due to the implementation of BlockManager.computeReplicationWorkForBlocks; node.getNumberOfBlocksToBeReplicated() remains zero given node.addBlockToBeReplicated is called after source node iteration.
4. So several decomm-in-progress nodes A, B, C end up with 4000 node.getNumberOfBlocksToBeReplicated().
5. If we assume each node can replicate 5 blocks per minutes, it is going to take 800 minutes to finish replication of these blocks.
6. Pending replication timeout kick in after 5 minutes. The items will be removed from the pending replication queue and added back to neededReplication. The replications will then be handled by other source nodes of these blocks. But the blocks still remain in nodes A, B, C's pending replication queue, DatanodeDescriptor.replicateBlocks, so A, B, C continue the replications of these blocks, although these blocks might have been replicated by other DNs after replication timeout.
7. Some block' replicas exist on A, B, C and it is at the end of A's pending replication queue. Even though the block's replication timeout, no source node can be chosen given A, B, C all have high pending replication count. So we have to wait until A drains its pending replication queue. Meanwhile, the items in A's pending replication queue have been taken care of by other nodes and no longer under replicated.