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https://issues.apache.org/jira/browse/HADOOP-5034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12664730#action_12664730
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Hairong Kuang commented on HADOOP-5034:
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I do not think fine-tuning the limit will solve the problem. What happened was
that block replication starved block deletion. Block deletion was not observed
at all. When I ReplicationMonitor code, I found out that block deletion does
not get scheduled for any datanode even if there is only one replication work
scheduled for the whole cluster. This explains why no block deletion was
observed at all.
> NameNode should send both both replication and deletion requests to DataNode
> in one reply to a heartbeat
> --------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-5034
> URL: https://issues.apache.org/jira/browse/HADOOP-5034
> Project: Hadoop Core
> Issue Type: New Feature
> Components: dfs
> Reporter: Hairong Kuang
> Fix For: 0.21.0
>
>
> Currently NameNode favors block replication requests over deletion requests.
> On reply to a heartbeat, NameNode does not send a block deletion request
> unless there is no block replication request.
> This brings a problem when a near-full cluster loses a bunch of DataNodes. In
> react to the DataNode loss, NameNode starts to replicate blocks. However,
> replication takes a lot of cpu and a lot of replications fail because of the
> lack of disk space. So the administrator tries to delete some DFS files to
> free up space. However, block deletion requests get delayed for very long
> time because it takes a long time to drain the block replication requests for
> most DataNodes.
> I'd like to propose to let NameNode to send both replication requests and
> deletion requests to DataNodes in one reply to a heartbeat. This also implies
> that the replication monitor should schedule both replication and deletion
> work in one iteration.
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