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From "Uma Maheswara Rao G (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4504) DFSOutputStream#close doesn't always release resources (such as leases)
Date Wed, 21 Aug 2013 07:42:56 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13745848#comment-13745848

Uma Maheswara Rao G commented on HDFS-4504:

Looks like this is good problem scenario pointed by Vinay here.
Client should not call complete without knowing that he receives ack for last packet. Otherwise
DN might be there with RBW state block and NN block state may become committed as client called
to complete. Now states will become inconsistent and internalRecoverLease also will not be
allowed because committedBlock did not get minreplicas reported by DN.

      // Close file if committed blocks are minimally replicated
      if(penultimateBlockMinReplication &&
          blockManager.checkMinReplication(lastBlock)) {
        finalizeINodeFileUnderConstruction(src, pendingFile,
            iip.getLatestSnapshot(), false);
          + " internalReleaseLease: Committed blocks are minimally replicated,"
          + " lease removed, file closed.");
        return true;  // closed!
      // Cannot close file right now, since some blocks 
      // are not yet minimally replicated.
      // This may potentially cause infinite loop in lease recovery
      // if there are no valid replicas on data-nodes.
      String message = "DIR* NameSystem.internalReleaseLease: " +
          "Failed to release lease for file " + src +
          ". Committed blocks are waiting to be minimally replicated." +
          " Try again later.";

ideally block is committed means, DN must have finalyzed also. SO, DN will report finalysed
block state in that case. Here, DN has RBW state only as it was failed in between. Due to
that failure, it got added to zombie and it will try to complete the file without knowing
whether he receives really last packet ack or not.

In normal recovery case, block will be finalysed by normal recovery flow as below:

      final BlockInfoUnderConstruction uc = (BlockInfoUnderConstruction)lastBlock;
      // setup the last block locations from the blockManager if not known
      if (uc.getNumExpectedLocations() == 0) {
      // start recovery of the last block for this file
      long blockRecoveryId = nextGenerationStamp(isLegacyBlock(uc));
      lease = reassignLease(lease, src, recoveryLeaseHolder, pendingFile);
      // Cannot close file right now, since the last block requires recovery.
      // This may potentially cause infinite loop in lease recovery
      // if there are no valid replicas on data-nodes.
                "DIR* NameSystem.internalReleaseLease: " +
                "File " + src + " has not been closed." +
               " Lease recovery is in progress. " +
                "RecoveryId = " + blockRecoveryId + " for block " + lastBlock);
this will make DN blocks to finalyze if they are in RBW state. But here if we change the state
already to committed, then recovery flow will be diverted and no one will finalyze the block
at DN. I am affraid that, this changes may cause the problems like this. So, better to do
recovery with NN only I think by just just informing the zombie files to NN when NN available.
Once we inform to NN successfully about zombie file successfully then we can remove such file
entries from his list. untill that try informing to NN about zombie files. This may be better
choice which may avoid the risks like above scenarios.
> DFSOutputStream#close doesn't always release resources (such as leases)
> -----------------------------------------------------------------------
>                 Key: HDFS-4504
>                 URL: https://issues.apache.org/jira/browse/HDFS-4504
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>         Attachments: HDFS-4504.001.patch, HDFS-4504.002.patch, HDFS-4504.007.patch, HDFS-4504.008.patch,
HDFS-4504.009.patch, HDFS-4504.010.patch, HDFS-4504.011.patch, HDFS-4504.014.patch, HDFS-4504.015.patch,
> {{DFSOutputStream#close}} can throw an {{IOException}} in some cases.  One example is
if there is a pipeline error and then pipeline recovery fails.  Unfortunately, in this case,
some of the resources used by the {{DFSOutputStream}} are leaked.  One particularly important
resource is file leases.
> So it's possible for a long-lived HDFS client, such as Flume, to write many blocks to
a file, but then fail to close it.  Unfortunately, the {{LeaseRenewerThread}} inside the client
will continue to renew the lease for the "undead" file.  Future attempts to close the file
will just rethrow the previous exception, and no progress can be made by the client.

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