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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (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, 06 Feb 2018 22:53:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10453?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16354677#comment-16354677
] 

Arpit Agarwal commented on HDFS-10453:
--------------------------------------

Hi [~hexiaoqiao], I assume this is the key part of the fix in the v7 patch:
{code}
    // Skip choose targets for block where one of the following conditions:
    //  a. additional number of replicas wanted is zero
    //  b. the datanode number of cluster is zero
    //  c. block has been deleted which is indicated by BlockCommand.NO_ACK.
    if (numOfReplicas == 0 || clusterMap.getNumOfLeaves()==0
            || blocksize == BlockCommand.NO_ACK) {
      return DatanodeStorageInfo.EMPTY_ARRAY;
    }
{code}
i.e. chooseTargets avoids looking for replication targets if the blockSize was changed to
NO_ACK.

This won't work. The block.length field was read by the caller ReplicationWork#chooseTargets
after releasing the lock, so there's no guarantee it sees the most recent value. I don't think
we should attempt to fix chooseTarget at all since it runs outside the lock.

Your v4 patch was on the right track. I will review it more closely.

Also we'll need a trunk patch (and patches for branch-2.9 and branch-2.8, if we want to commit
this to branch-2.7).

> 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: 2.7.6
>
>         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-10453.001.patch
>
>
> 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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