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From "He Xiaoqiao (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-10453) ReplicationMonitor thread could stuck for long time due to the race between replication and delete of same file in a large cluster.
Date Tue, 24 May 2016 07:14:13 GMT
He Xiaoqiao created HDFS-10453:
----------------------------------

             Summary: ReplicationMonitor thread could stuck for long time due to the race
between replication and delete of same file in a large cluster.
                 Key: HDFS-10453
                 URL: https://issues.apache.org/jira/browse/HDFS-10453
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: namenode
    Affects Versions: 2.7.1
            Reporter: He Xiaoqiao


ReplicationMonitor thread could stuck for long time and loss data with little probability.
Consider the typical scenarioļ¼š
(1) create and close a file with the default replicas(3);
(2) increase replication (to 10) of the file.
(3) delete the file while ReplicationMonitor is scheduling blocks belong to that file for
replications.

if ReplicationMonitor stuck reappeared, NameNode will print log as:
{code:xml}
2016-04-19 10:20:48,083 WARN org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy:
Failed to place enough replicas, still in need of 7 to reach 10 (unavailableStorages=[], storagePolicy=BlockStoragePolicy{HOT:7,
storageTypes=[DISK], creationFallbacks=[], replicationFallbacks=[ARCHIVE]}, newBlock=false)
For more information, please enable DEBUG log level on org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy
......
2016-04-19 10:21:17,184 WARN org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy:
Failed to place enough replicas, still in need of 7 to reach 10 (unavailableStorages=[DISK],
storagePolicy=BlockStoragePolicy{HOT:7, storageTypes=[DISK], creationFallbacks=[], replicationFallbacks=[ARCHIVE]},
newBlock=false) For more information, please enable DEBUG log level on org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy
2016-04-19 10:21:17,184 WARN org.apache.hadoop.hdfs.protocol.BlockStoragePolicy: Failed to
place enough replicas: expected size is 7 but only 0 storage types can be selected (replication=10,
selected=[], unavailable=[DISK, ARCHIVE], removed=[DISK, DISK, DISK, DISK, DISK, DISK, DISK],
policy=BlockStoragePolicy{HOT:7, storageTypes=[DISK], creationFallbacks=[], replicationFallbacks=[ARCHIVE]})
2016-04-19 10:21:17,184 WARN org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy:
Failed to place enough replicas, still in need of 7 to reach 10 (unavailableStorages=[DISK,
ARCHIVE], storagePolicy=BlockStoragePolicy{HOT:7, storageTypes=[DISK], creationFallbacks=[],
replicationFallbacks=[ARCHIVE]}, newBlock=false) All required storage types are unavailable:
 unavailableStorages=[DISK, ARCHIVE], storagePolicy=BlockStoragePolicy{HOT:7, storageTypes=[DISK],
creationFallbacks=[], replicationFallbacks=[ARCHIVE]}
{code}

This is because 2 threads (#NameNodeRpcServer and #ReplicationMonitor) process same block
at the same moment.
(1) ReplicationMonitor#computeReplicationWorkForBlocks get blocks to replicate and leave the
global lock.
(2) FSNamesystem#delete invoked to delete blocks then clear the reference in blocksmap, needReplications,
etc. the block's NumBytes will set NO_ACK(Long.MAX_VALUE) which is used to indicate that the
block deletion does not need explicit ACK from the node. 
(3) ReplicationMonitor#computeReplicationWorkForBlocks continue to chooseTargets for the same
blocks and no node will be selected after traverse whole cluster because  no node choice satisfy
the goodness criteria (remaining spaces achieve required size Long.MAX_VALUE). 

During of stage#3 ReplicationMonitor stuck for long time, especial in a large cluster. invalidateBlocks
& neededReplications continues to grow and no consumes. it will loss data at the worst.

This can mostly be avoided by skip chooseTarget for BlockCommand.NO_ACK block and remove it
from neededReplications.



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