Troubleshooting Performance of Decommissioning

Several conditions can impact performance when you decommission DataNodes.

Open Files
Write operations on the DataNode do not involve the NameNode. If there are blocks associated with open files located on a DataNode, they are not relocated until the file is closed. This commonly occurs with:
  • Clusters using HBase
  • Open Flume files
  • Long running tasks
To find open files, run the following command:
hdfs dfsadmin -listOpenFiles -blockingDecommission
The command returns output similar to the following example:
Client Host         Client Name         Open File Path        DFSClient_NONMAPREDUCE_-698274460_1 /hbase/oldWALs/

After you find the open files, perform the appropriate action to restart process to close the file. For example, major compaction closes all files in a region for HBase.

Alternatively, you may evict writers to those decommissioning DataNodes with the following command:
hdfs dfsadmin -evictWriters <datanode_host:ipc_port>
For example:
hdfs dfsadmin -evictWriters datanode1:20001
A block cannot be relocated because there are not enough DataNodes to satisfy the block placement policy.
For example, for a 10 node cluster, if the mapred.submit.replication is set to the default of 10 while attempting to decommission one  DataNode, there will be difficulties relocating blocks that are associated with map/reduce jobs.    This condition will lead to errors in the NameNode logs similar to the following:   
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault: Not able to place enough replicas, still in need of 3 to reach 3   
Use the following steps to find the number of files where the block replication policy is equal to or above your current cluster size:
  1. Provide a listing of open files, their blocks, the locations of those blocks by running the following command:
    hadoop fsck / -files -blocks -locations -openforwrite 2>&1 > openfiles.out
  2. Run the following command to return a list of how many files have a given replication factor:
    grep repl= openfiles.out | awk '{print $NF}' | sort | uniq -c
    For example, when the replication factor is 10 , and decommissioning one:
    egrep -B4 "repl=10" openfiles.out | grep -v '<dir>' | awk '/^\//{print $1}'
  3. Examine the paths, and decide whether to reduce the replication factor of the files, or remove them from the cluster.