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https://issues.apache.org/jira/browse/HDFS-10453?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16924753#comment-16924753
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He Xiaoqiao commented on HDFS-10453:
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[~hustnn] I think this is explained case, when some datanodes are shutdown at
the same time, NameNode will trigger to replicate under-replication blocks,
then some live datanode will be high load especially for small cluster. I just
wonder if namenode process meet some exception or hung long times when log
above information?
> 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: 2.4.1, 2.5.2, 2.7.1, 2.6.4
> Reporter: He Xiaoqiao
> Assignee: He Xiaoqiao
> Priority: Major
> Fix For: 3.1.0, 2.10.0, 2.9.1, 2.8.4, 2.7.6, 3.0.3
>
> Attachments: HDFS-10453-branch-2.001.patch,
> HDFS-10453-branch-2.003.patch, HDFS-10453-branch-2.7.004.patch,
> HDFS-10453-branch-2.7.005.patch, HDFS-10453-branch-2.7.006.patch,
> HDFS-10453-branch-2.7.007.patch, HDFS-10453-branch-2.7.008.patch,
> HDFS-10453-branch-2.7.009.patch, HDFS-10453-branch-2.8.001.patch,
> HDFS-10453-branch-2.8.002.patch, HDFS-10453-branch-2.9.001.patch,
> HDFS-10453-branch-2.9.002.patch, HDFS-10453-branch-3.0.001.patch,
> HDFS-10453-branch-3.0.002.patch, HDFS-10453-trunk.001.patch,
> HDFS-10453-trunk.002.patch, HDFS-10453.001.patch
>
>
> ReplicationMonitor thread could stuck for long time and loss data with little
> probability. Consider the typical scenarioļ¼
> (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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